"Andy Warhol is way overvalued," said New York art dealer Richard Feigen, whose clients include Paris's Louvre museum. "Jeff Koons is vastly overvalued. Damien Hirst is top of the list." The world's biggest-ever contemporary art auctions start tonight, with pop artist Warhol's $35 million image of Elizabeth Taylor and Koons's $20 million "Hanging Heart" among the lots. The sales at Christie's International and Sotheby's will help gauge the extent of a slowdown in the market. Feigen, an adviser to former hedge-fund manager Michael Steinhardt, said collectors who sought investment profit "may walk away" if things go wrong. "People in sweatshirts have been throwing millions around," Feigen said in an interview. "A great deal of buying is done as speculation, and if a work by Koons loses 50 percent, he'll be dumped for something with greater sex appeal." Sotheby's shares fell 38 percent in two days after the second-largest auction house missed its low estimate at a Nov. 7 Impressionist art sale and said it lost $14.6 million on guaranteed works, for which it must pay sellers. The result indicates an 11-year bull run in art may be cresting after losses in global markets tied to the slump in subprime debt. This week's $914 million combined top estimate of Christie's International, Sotheby's and Phillips de Pury & Co. represents twice as much art as the world's biggest contemporary fair, Switzerland's Art Basel, traded in June, and would beat May's auction total. Feigen, 77, an agile, square-jawed man with spectacles, has sold works spanning six centuries to 110 museums, including London's National Gallery and Los Angeles' J. Paul Getty Museum in the past 50 years. Orchids and Old Masters An early dealer in Francis Bacon, Claes Oldenburg and James Rosenquist, he still trades modern and contemporary art while accumulating Old Masters. They hang in his office, filled with books, leather couches and orchids in a 1929 low-rise building on East 69th Street. "He's a very knowledgeable dealer," said Steinhardt. High prices and guarantees make the contemporary auctions especially risky, said Feigen. Sotheby's and Christie's have made about $360 million of guarantees for their evening sales, said analyst Kristine Koerber of JMP Securities. Guarantees, or promises of minimum prices to sellers, transfer the risk of a drop in art market to the auction house. Among the top lots that are guaranteed, Sotheby's on Nov. 14 has Warhol's "Self-Portrait" and Koons's "Hanging Heart," while Christie's sale tomorrow has Warhol's "Liz" and Koons's "Diamond (Blue)." "It could have a ripple effect" on collectors' confidence if guaranteed items fare poorly, Feigen said. "Overpriced and Overrated" Britain's Hirst, whose spot painting "Naja Naja Sputatrix" has a $1.5 million top estimate at Christie's this week, is "overpriced and overrated," Feigen said. So are Japan's Takashi Murakami, whose art extends to placemats, and Italy's Maurizio Cattelan, known for his Pope John Paul II waxwork. Marlene Dumas, Tom Wesselmann, Mel Ramos and Jean-Michel Basquiat are also on Feigen's list of the overpriced. "Except for Warhol, they will not have a place in art history," Feigen said. On Feigen's wall is a 1557 Frans Floris painting, "Arithmetica." One of Antwerp's most important artists, Floris will always be in demand by museums, Feigen said. "It's worth about $850,000," he said. "That wouldn't get you a minor Basquiat" -- a graffiti artist who died in 1988 at age 27. Basquiat's 1982 painting of boxer "Sugar Ray Robinson" has a top estimate of $8 million at Christie's tomorrow. Feigen recently bought a painting by 15th-century Italian artist Fra Angelico, whose works hang in museums from Paris to Boston. "I had to have it and I have no doubt about its place in art history." 1990s Slump Many artworks lost half their value in the five years after the market crashed in 1990, according to index-maker Art Market Research. "New buyers have only seen escalation," Feigen said. "I've been in salesrooms where it has been 'pass, pass, pass.'" Still, last week's poor Impressionist sales may not indicate another slump is coming, said Cristin Tierney, a New York art adviser. "It's too early to say there is a correction," he said. "But in an uncertain financial market, people are going to look before they leap." New York dealer Andrew Fabricant of Richard Gray Gallery said the stumble in sales may force auction houses and sellers to cut prices for second-tier works. "It's a wake-up call that you cannot sell B-minus material for A-plus prices," he said. Sotheby's has guaranteed about 78 percent of the low value of its Nov. 14 contemporary sale, up from 35 percent last year, and Christie's has commitments for 52 percent of its Nov. 13 sale, according to analyst Koerber. Christie's values its contemporary sales this week at as much as $473.4 million, while Sotheby's high estimate is $393.3 million. Phillips's Nov. 15 auction has a top estimate of $47.6 million. Last week's Impressionist auctions at Christie's and Sotheby's took in $804 million. Feigen's partial list of overvalued artists: Damien Hirst Jeff Koons Andy Warhol Jean-Michel Basquiat Takashi Murakami Maurizio Cattelan Mel Ramos Tom Wesselmann Marlene Dumas