The newest and biggest nightclub in Washington opened last night neither with a whimper nor a bang but with a collective sigh of relief from several dozen employes for whom the waiting was over, and suddenly they were serving a thousand customers. The dolphin tank has been covered with a stage, and the curtain has closed for the last time on Snidely Whiplash. The historic figures that once lined the walls have been put away in wax balls but the name lingers on--The Wax Museum, um, Nightclub.

Folk singer Tom Paxton, who opened for '60s folk hero Arlo Guthrie, surveyed the college-plus crowd and said, "I've closed a lot of clubs. It sure is nice to be opening one for a change." Surprisingly, there was no hoopla, no ribbon cutting, no bottles of champagne busting on the edge of the stage. Where there had not been a nightclub Wednesday night, there suddenly was on Thursday. You'd almost think it happened every Thursday.

For those who packed into the 1,000-seat club at 4th and E streets SW the mood of the evening was as mellow as a Woodstock anniversary party thrown by a generation bent on remembering the good times. Guthrie, who seemed thrilled by the ambiance of the new club, gave passionate readings of such favorites as "City of New Orleans" and several of his patented talking blues.

The confused buzz of the morning's last-minute inspections and hurried preparations--cables were crossed, headphones connected, tables aligned--was replaced at show time by the buzz of first-nighters sharing a collective opening night.

And despite a 45-minute delay due to last-minute communications problems, the club received good grades from its most important critics--the fans. "This is just swell," gushed Lannie Preston, a government clerk who drove in from Fairfax with several friends for the event. "I can't believe this is their first night. Come to think of it, I think I've been here before. But I don't think so."

"I've been waiting for a room like this," added Ted Downs, a University of Maryland student who's been following Guthrie for 10 years. "This is such a nice way to hear and see him. Usually I'm half a mile away."

Guthrie's set, laden with songs from Father Woody and friend Bob Dylan, bustled with the sudden community of '60s interest being rekindled in the '80s. After Paxton's opening set of topical songs aimed at joggers, Abscam, Rubik's Cube, and smart bombs, Guthrie's frail vocal insistence and semi-political humor seemed to inspire the crowd to lighthearted enthusiasm. They also came hungry for more than music--the kitchen was inundated with 60 dozen orders for "Delicious Potato Skins," apparently a new specialty for music lovers.

Early yesterday morning, with all the dummies from the old Wax Museum tucked out of sight, it was decision-making time about who to bring out of the closet. The choices: Louis Armstrong, Duke Ellington, John Phillip Sousa, John Wayne.

"I don't think we'll be using Jonas Salk or Hiram Fong," said Keith Krokyn, the club's talent and booking manager. "We do have pieces of the Beatles, like Paul McCartney's head and lower torso."

"Paul's and Ringo's heads," added Joe Fab, assistant general manager for Historic Figures Inc., the club's owners. "With dummies, it's the heads that count." Fab, Krokyn and everyone else will be counting heads from now on as the Wax Museum seeks to establish itself as the premier nightclub in the Washington area. They started out with a bang, selling out the Guthrie show and turning away scores of would-be customers.

With state-of-the-art sound, lights, video and recording facilities, the Wax Museum should have a major impact on Washington night life. Its size will allow it to book many acts whose popularity or cost keep them out of the smaller clubs or the slightly larger (and decidedly more expensive) concert halls. Upcoming concerts include country crooner George Jones (tomorrow night), Al DiMeola, Leon Russell, the Four Tops, Ricky Lee Jones, John McLaughlin, Tina Turner and the Pointer Sisters.

The spacious club, located three blocks off the Mall, certainly has a lot going for it, including a 1,000-car garage connected to the nightclub (Historic Figures Inc., is owned by Colonial Parking). There is also a pair of subway stops two blocks away (L'Enfant Plaza and Federal Center SW). The sound system, run by Dude Sless (recently of the Bayou) has been installed by Maryland Sound of Baltimore, which has provided systems for the Grateful Dead, Frank Zappa, Al Jarreau and Hall and Oates. Seating is in three levels: 400 seats sunk into the ground around a thrust stage, and the rest spread among two sections of long tables, the back ones set on risers. There is a reasonable dance space to the front of the stage that won't interfere with viewers' sightlines.

There will be extensive video programming and special events taking advantage of the 70-foot-long stage (the D.C. Emmies, hosted by Bryant Gumbel and Andy Rooney, will be held there in June). Guthrie's performance last night was videotaped for syndication and Gil Scott-Heron's birthday concert on April 1 will be filmed for British television. A major video production company is expected to move into the building very soon.

The Southwest building has a history as checkered as it is short. Once a turning station for the trolley system, it became home to the Wax Museum in 1975 when it was displaced by the government from its longtime New York Avenue home. The new location was quite good, only three blocks from the Mall and the Smithsonian museums. The complex, totally geared toward the tourism trade, housed a cafeteria and gift shop and, since "something was needed as a package for tourists, they put in performing dolphins," Fab explained.

The dolphins left in 1977 ("they became ill indoors") and several "extras" followed: "American Adventure," a multi-media bicentennial show once housed in the basement of the Warner Theatre, lasted until last spring; Gateway Dinner Theatre, dedicated to American meldorama, lasted a little over a year, closing this past Christmas. Unfortunately, Fab says, wax museums have lost their appeal and tourism, besides being seasonal, was not consistent. "It's great to be three blocks from the Mall but when everything three blocks away is free and you're a private enterprise, it's hard for people from out of town to get that clear. They think that anything that isn't dinner or where they sleep is going to be free."

The metamorphoses from museum Museum to nightclub Museum began last fall, when Rich Vendig, owner of Desperado's in Georgetown, approached Historic Figures Inc.; the closing of Gateway removed the final block. Vendig was unable to find sufficient backers but Historic Figures went ahead with the nightclub project on its own with Vendig as operations manager. "People have been calling for the last three weeks, asking 'What's playing tonight?' " says Krokyn. "It's as if we were already rolling." Krokyn and company think that's a great way to start a business. By midnight, it was decidely good news.