Marilyn Lewis showed up at the Corcoran Gallery last night with one thing in mind: "I want a real good piece of art." So did about 600 other people at the gallery's third annual Affordable Art Auction--but Lewis wouldn't even contemplate defeat.

What did Lewis, chairman of the board of the Hamburger Hamlet restaurant chain, have her eye on? "Oh, it's a secret . . . well, actually, the Bonnard," an 1895 lithograph that was a widespread favorite among aficionados.

Two and a half hours later, after an aggressive three-way shoot-out, Lewis had her prize and received a polite round of applause for the $2,500 triumph, the evening's top sale.

"We did it!" Lewis said with a proud laugh. "I'm going to hang it in the Georgetown Hamlet; what do you think of that?"

Lewis' enthusiasm, if not her decorating theories, was typical at the festive auction held as a benefit for the Corcoran School of Art. Veteran collectors mingled casually with wide-eyed rookies in the gallery's elegant main atrium before the bidding began, admiring works donated by prominent local artists and longtime Corcoran patrons.

Art school dean Bill Barrett estimated the event raised more than $45,000.

"These things are terrific," commented CBS correspondent Bob Schieffer, who was one of the many people who bought a $5 entrance ticket just to look. "I came mainly just to see what's out and about," Schieffer explained. "You know I'm an artist in addition to the other things I do, so I always like to see what people are doing and what prices run."

Though the prebidding atmosphere in the central hall was highly civilized, an adjacent room of about 200 lesser pieces resembled a Sears bargain basement during a lawn-care equipment clearance.

Jostling to get a good view of the decidedly eclectic exhibition, a younger crowd in more casual summer dress fought to get their personal art empires off on the right foot.

Patricia O'Brien and John Lescroant, both 27, recently moved into their new apartment, "and it's going to be our first piece of real art, you know how it is," said O'Brien. "There's absolutely nothing on the walls; we've got to get something," added Lescroant, confirming the couple's interest in the under-$100 range.

Auctioneer Dale W. Stulz, on loan from Christie's gallery in New York City, stressed that affordable art does not take the fun out of an auction: "It's like any of the biggest, most expensive affairs--it's a little bit of theater, a little bit of psychology and a little bit of poker."

"Everyone is so excited," agreed Olga Hirshhorn, who introduced the low-price auctions two years ago and was cochairman this year's event. "It's a great thing for the school . . ."

Frequently coaxing his victims with reminders that they were "doing a good thing for the Corcoran," Stulz managed to move many items that were neither particularly cheap nor immediately impressive as investments.

Bidding started low on a Noche Crist work titled "Candelabra," but built to $350--not bad for a pair of gaudy pink-and-green candlesticks fashioned in the shape of nude female torsos. The winner in that battle, however, was exultant. "Wow, they're great!" said Dixonson Hoffman. "I just bought a new house, and it's going to have a very interesting dressing room, and I'll put these in it. Olga Hirshhorn has a pair, have you seen them?"

Not everyone, however, was thrilled with the idea of a populist art auction. "What was I looking for?" pondered collector Jim Trulove. "I was looking for a rainy night when no one else would show up and I could get some good deals on good art." Looking around at the happy crowd, he added, "It just didn't happen."