Once merely the best little boogie band in Texas, ZZ Top has become one of the absolute biggest bands in the entire U.S. of A., thanks to blues-based, southwestern-fried riff rock, the 20-inch beards of guitarist Billy Gibbons and bassist Dusty Hill (drummer Frank Beard has the right name but only a mustache), and an image so eccentrically videokinetic you'd think the band had been invented for MTV.

Actually, ZZ Top predates it by a dozen years, but it was MTV and the music/video revolution that broadened the trio's appeal by bringing what had been a cult band -- albeit one with a huge following -- into Everyteen's living room with the message that rock 'n' roll could set you free, particularly if ZZ Top gave you the keys to Gibbons' $100,000 1933 two-door coupe, the red, hot Eliminator. Instead of a spare tire, you got three leggy Texas belles who'd stepped right out of a dream.

Who could resist that promise?

And now, who can forget the Smith Brothers of Boogie?

"If you want to have a good time, just come out and partake in the fun," Gibbons says. "We're humorous, short of comical -- nobody's left holding the bag. It's more, 'Hey, if you want to have a good time, then it's there for the taking.' "

Saturday night at Capital Centre, there were 17,000 takers, as there were again last night and as there will be tonight and tomorrow night. In all, ZZ Top sold out four Cap Centre shows, though several thousand extra tickets will be available Monday and Tuesday nights for seats freed up because the production setup took less space than expected.

Not surprisingly, it's a Texas-sized show, built on 100,000 pounds of sound, light and laser equipment hauled by the four 48-foot semis parked outside. The tech crew alone numbers 23.

"All of a sudden we're these Big Famous Guys," says the hirsute Gibbons. "People expect a whole, whole lot, and we're prepared to deliver."

Looking like oversized recruits to Snow White's bodyguard staff, Gibbons and Hill are ensconced in a visitors' dressing room at Capital Centre, unwinding after their first show in two weeks, a powerful 90-minute set redolent with the blues-based boogie that always has been the heart of their music.

"I went back and studied our first two albums, which were 15, 16 years ago," Gibbons says, and "the music hasn't really changed all that much. I don't think we sing as much about Texas as we used to, but we're up there playing the same three chords. It's pretty much the same."

Actually, it's gotten much better, and the audiences have gotten much bigger: ZZ Top is 71 dates into a 220-concert tour that will crisscross America before taking it around the world and will ultimately attract about 4 1/2 million fans. With its souped-up dashboard decor, laser effects and spectacular lights, this tour is proof that Texans can not only do things big, but well.

"Designers today have economics in mind," Gibbons says, "and that's making it possible to take this kind of entertainment back to the road."

Ironically, ZZ Top used to do things even bigger. A decade back, "this kind of entertainment" meant a 3,000-square-foot, Texas-shaped, cacti-and-corral-cluttered stage holding 150,000 pounds of equipment -- housed in eight semis, it took a full day to set up, a full day to take down. And that stage was home not only to the group but to a menagerie that included rattlesnakes (they eventually overdosed on the vibrations), a coyote (his howl used to introduce the show, but he got lazy and had to be replaced with a recording), four tethered buzzards, a longhorn steer and a buffalo ("you did not want to walk up on his backside," Gibbons notes). There was even a pig, though he rarely left the dressing room, apparently preferring the backstage zoo to the on-stage one.

Show business being so demanding, ZZ Top had to rotate its buzzards, and also had to remember to stay out of their reach. "You know what a buzzard's defense mechanism is?" Gibbons asks "They womit on you. And that is bad stuff -- it can be really raunchy."

With the ASPCA keeping tabs on the traveling zoo, he adds, "the buffalo and the longhorn got treated better than we did," grazed every 10 hours and housed in great comfort. "They kept that from us for a long time, until one day I said, 'What's that air-conditioned truck with all those people?' and they said, 'That's the attendants to the buffalo, and the attendants to the longhorn . . .' "

Once the longhorn got loose and somehow ended up perched on top of two backstage limousines parked end to end. They buckled under his weight, which must have provoked some interesting answers on the insurance report.

"Back in those days everybody had a giant show out, and it was who could top the next guy," Gibbons recalls, if not fondly. "But that kind of stopped about '76, and that's when we went off the road."

Indeed, at what most people considered the peak of its career, ZZ Top disappeared for three years. Of course, it turned out to be just a peak, not The Peak.

"Everybody thought we were crazy for taking a break," says bassist Hill. "It didn't start out to be three years, it started out to be six months, but we'd been 300 days a year on the road forever, and we just needed it . . .

"And that's when I started growing my beard long. Didn't think anything of it; it just started coming out. And Billy was over in Europe, doing the same thing."

With their long, flowing beards, Gibbons and Hill looked like hip Van Winkles, or Howard Hughes' long-lost brothers. "When we laid eyes on each other, I thought, that'll work," Gibbons says. "So we left it and it became our thing. Our image was so obtuse to begin with that I thought, hell, if this ever goes over, who cares?

"It's interesting, because this oddball appearance has not only become uniquely ours, but it was not offensive. We figured, man, they're going to think we're a bunch of hicks or they're going to think we're selling cough drops again . . . But we haven't had too much trouble from it, though it is hard to go shopping . . . "

How about tucking the beards inside shirts or sweaters? "Don't work," Gibbons sighs. "I've tried it. They still recognize us."

Of course, it wasn't the beards that took the band to a higher ground after years of packed concerts and platinum record sales -- it was the videos of the beards that supplemented 1983's "Eliminator" album. MTV latched onto ZZ Top, followed quickly by new, young, mixed audiences latching onto ZZ Top. Videos for "Legs," "Sharp Dressed Man," "Gimme All Your Lovin'," "Cheap Sunglasses" and more recently the high-tech, surreal "Rough Boy" have made the band as visible as it had once been audible.

"We've been legitimized to a wider spectrum of people because they can relate to us now," Gibbons says. "We were a pretty cultish kind of thing. We had big audiences, but it was reserved to a much more confined grouping of folks," mostly young white males who found a macho referent in ZZ Top's outlook on life, which could be summed up as roaring down an interstate in the Eliminator, beer in hand, looking for the proverbial good time had by all.

"Now we can expect to see everybody," Gibbons says, apparently relieved.

A good part of that everybody seems to be younger, Hill adds, "at least by comparison to us. Starting with 'Eliminator,' we still had our audience, our age, but we also started getting 14-, 15-, 16-year-old kids besides 'em."

And for the first time the band started attracting a healthy percentage of girls. "No kidding," Hill smiles. "Man, I'm glad to see that. I was afraid our reputation was going to start turning weird."

ZZ Top's history is not particularly weird, but it is longer than those whiskers. All three were consumed with music early on, Hill playing with blues great Freddie King at the tender age of 14. He and Beard were in a Dallas band, American Blues (they dyed their hair accordingly), and Gibbons, whose father was a pianist and occasional conductor of the Houston Philharmonic, first made a name for himself in a semilegendary '60s psychedelic band, the Moving Sidewalks. They eventually gravitated to their optimum lineup, becoming ZZ Top 16 years ago.

The core of their music was a fusion of raw blues and electric boogie, and for many years it was aimed at a blue-collar crowd defined and celebrated in one song as "Beer Drinkers and Hell Raisers." But it also explored Texas landscapes as disparate as "Rio Grande Mud" and "La Grange," the whorehouse later made famous by Larry King as the best little one of its kind in Texas. A southwestern mythology was also at work, evident in album titles such as "Tres Hombres" and "Deguello" ("Spare No Quarter," the order given by Gen. Santa Ana before the final assault on the Alamo, where two of Hill's ancestors perished).

Of course, some thought ZZ Top's music spared no quarter, either, and while it was for years unappreciated by critics, the band developed a huge populist following with meaty, infectious anthems such as "Arrested While Driving Blind," "Tube Snake Boogie" and the bar-band classic, "Tush."

The music was faithful to the roots and spirit of the blues, yet original. Unlike many Texas guitarists, Gibbons relied less on speed and bravado than on feeling and effect -- his playing was technically polished but soulfully raw, his solos economical yet searing. The whole band is so tight it can stop and start on a dollar (somehow you can't imagine Texans stopping on a dime).

Which doesn't mean ZZ Top has stopped evolving. Over the past few years, it's incorporated prerecorded "click tracks" to underscore the basic pulse, though some critics have complained that the band has bowed to the rigid hit-making formula of 120 beats per minute.

"It doesn't really matter what speed you play as long as you play in good time," Gibbons says. "We're really not doing anything different from the studio guys in Los Angeles, it just helps you stay in line. That's the odd thing about rock 'n roll -- it's such a rebellious form by tradition. When somebody suggested we use a metronome and keep our time together and keep our tempo straight, it wasn't that big a discovery -- the metronome has been around since the invention of the clock. For that reason, it shouldn't be blown up to the proportion where we've 'cracked' this big code." The band's airplay has nevertheless increased since that change.

"What doesn't change is the human condition," Gibbons says. "And the human condition responds to regularity. The closer you get to perfect time, the better the body feels. It becomes trusting, and with trust comes relaxation, and with relaxation comes feeling good . . .

"It's about feeling good."

And looking good. Through the years, ZZ Top's image has kept pace with the music. In the early '70s it was a Bourbon Cowboy look, and the beginning of a parade of hats ranging from ten-gallons, top hats and Stetsons to berets, baseball caps, sombreros and bowlers. (Some scalawags suggest these hats cover hairlines that may be receding in proportion to the southern forays of Gibbons' and Hill's beards.) Still, it's the beards that people notice. Even some of the guitars wear them!

In 1974 the band started drifting to hip fashions, and an ongoing association with Manuel Cuevas, brother-in-law of the fabled Nudie. When it finally recorded "Sharp Dressed Man" in 1983, it was more autobiography than fantasy. Now, 16 years after making its first appearance in a Houston juke joint, ZZ Top has emerged as the unlikeliest of teen and fashion idols.

"I'm a sex symbol," Hill suggests with an incredulous laugh. "Is that wild? It blows me away. I mean, I've always known it, I just figured nobody else did."

For Gibbons and Hill, anonymity is just a close shave away, but neither sees another sabbatical coming up soon.

"If I get away from it for a week or so I get itchy," Hill says. "I'm always ready to go back out."

"I remember the itch, too," says Gibbons. "And don't think the music business doesn't sting you . . . Once you've tasted it, you want it."

Gibbons once said, "Unsaddle that pony or shoot it, but don't ride it into the ground." Today, ZZ Top is riding tall, and sharp-dressed, in the saddle.