She's only 20 years old, and if she's kept her shape, you can dress her up and sell her for $500.

She drives a Corvette, has a two-story house, tans, bends, sleeps, colors her hair and is just like any other young lady, with two exceptions:

She is a doll.

She is a collector's item worth about $500.

It's Barbie, that famous first "adult" doll bestowed upon an unsuspecting public 20 years ago.

An even though Mattel Toy Company, Barbie's maker, has sold about 112 million copies of Barbie and her friends, the older and rarer models of the doll are fast becoming valuable collectors' items -- and the latest hedge against inflation.

Last year, after a nationwide bidding war involving sealed bids and secret offers, one person sold a mint-condition, still-boxed original (circa 1959) Barbie Doll for $501.59.

Today, according to Sybil DeWein, Barbie expert and coauthor of The Encyclopedia of Barbie Dolls and Collectibles (Collection Books/Crown Publishers), the same doll would fetch considerably more money. But she cautioned, "I don't know of another one in existence."

And DeWein would know. She has 1,315 versions of Barbie or her friends, and all of the accessories to boot. According to DeWein, there are about 2,000 Barbie collectors around the world, and two monthly publications to fuel their interest -- Barbie Bulletin from Santa Ana, Calif., and the International Barbie Doll Collector's Gazette, published on City Island, in the Bronx, New York.

"Interest in Barbie has really accelerated in the past year and a half," says DeWein. "That has driven the prices way up."

Because the marketing whizzes at Mattel have made annual changes in the Barbie Doll products, collectors have been able to key on certain versions of the doll that were only produced one year or another.

According to Mattel's Beverly Kennedy, "We differentiate the product every year, but the body has always remained the same for technological reasons. If we dared change the body, it would mean that parents would have to go out and buy all new accessories, and we don't want the wrath of all the parents in the country to come down on us."

Still, Mattel has made changes. Barbie's hair style has changed three times in her 20 years, and facial features like eyebrows have changed to reflect the times.

And, there has been a whole collection of friends and relatives, not the least of whom is Ken, Barbie's boyfriend. Half of the 112 million dolls sold for between $3 and $10 have been Barbies, while the other half include replicas of Ken, little sister Skipper, cousin Francie and dozens of others.

Certain versions of Barbie have taken on special value to collectors. "There is the sleep-eyed Barbie that was only made in 1964," says DeWein. "That one closed its eyes when it lied down, but must have been too expensive to make because it only lasted a year." Sold for about $9.50, the sleep-eyed -- in good condition -- could bring between $35 and $65 today, DeWein said.

Other rare Barbies include the 1967 black friend called Francie, who was only made for one year and the series of friends sold in pairs in 1970, holding hands.

And even DeWein has been unable to find one particular Barbie, the 1965 bendable leg Barbie with her hair parted on the side. "Most of them were made with a center part," she said.

The varieties of Barbies and the accessories -- the Corvette called a "StarVette," the new two-story house (selling furnished for $100) and the 10-speed bike -- all make the new sport of Barbie collecting more interesting, and more expensive.

And the newest Barbie, the kissing Barbie, is bound to be popular. Press a panel on her back, and she puckers up, leaving lipstick stains on your collar.

But the real insight to Barbie's increasing popularity and marketing acceptance may have come from Barbie herself. In a rare 16th birthday interview with the Washington Post's Judith Martin four years ago, Barbie is quoted as saying:

"I feel there's nothing a girl can't do if she puts her mind to it, and of course, if she has the proper clothes."