"While you may think twice about buying a dress," said local art collector and businessman Ian Woodner, looking over toward Baron Hans Heinrich Thyssen-Bornemisza, "he'll think once about buying a painting."

The result is the baron's extraordinary collection of modern paintings by Picasso, Kandinsky, Mondrian, Chagall, Magritte and Hopper, to name a mere few. He has loaned 66 of his paintings for a tour of American museums that opens at the National Gallery on Sunday. And chances are he won't miss one of them, unless perhaps he happens to notice the Georges Braque gone from the wall of his 165-foot-yacht, which is usually moored in Monaco.

He is the collector's collector, something that was not lost on the well-heeled group of 200 local arts patrons, collectors, business people and members of Congress who gathered to fete him at a black-tie dinner last night at the gallery. Consider these assessments:

* "It's an example of what can happen when great wealth is combined with great taste," said collector David Lloyd Kreeger.

* "This isn't even the private collection," said Rep. Fred Richmond (D-N.Y.), who is a friend of the baron. "This is the contemporary collection, which tours. It's insured for $120 million. Plus, he has a collection of Old Masters, which tours. And he has a museum in Lugano [Switzerland]. And then you go to his house and you wouldn't think anything is missing."

* "These are paintings that every art critic knows and every art historian knows and has only seen in books," said Ray D'Argenio, senior vice president of United Technologies, the company underwriting the tour, the catalog and last night's dinner. "Now they can appear out of a guy's bedroom, kitchen and dining room. This is the second-largest collection of private paintings. The first is Queen Elizabeth's. That's pretty neat."

Among those who came were arts patron and chairman of the gallery's board of directors Paul Mellon, Secretary of Defense Caspar Weinberger, and the first lady's staff director, James Rosebush, and his wife Nancy.

The baron is one of those people who seems to have always been rich -- his father was heir to a German iron and steel fortune and the baron has holdings in European banking, shipping and shipbuilding, as well as owns an American industrial conglomerate called Indian Head. He has spent a lot of it on his paintings.

"Well, quite a lot last year, unfortunately," he said with an understated chuckle as guests wandered through the exhibit. But the baron doesn't simply write out a check every time. "Sometimes you can pay over a year, or you can do part exchange."

"I don't think that there is anyone living who has the passion for collecting that he does," said Woodner, himself no slouch at collecting. He has a Hans Holbein drawing once owned by the baron's father. Of the baron, he added, "he would never sell anything." Nor does he give it away, although many museums would be happy to have it all.

Agreed the baron: "Oh, yes. But I don't do that." Asked if he would ever donate anything to the National Gallery, he paused, smiled and folded his arms. "No," he said. "I don't think so." But the baron seemed pleased with the installation of his exhibit.

"Well, you make a wonderful thing again," he told gallery Director J. Carter Brown.

"Well, it's nice to have something great to work with," said Brown.

In the receiving line before the dinner of veal, the baron stood greeting guests, with his wife the Baroness Denise Thyssen-Bornemisza at his side. She is his fourth wife, Brazilian-born, younger than he, tan with long blond hair, and diamond jewelry around her neck. Also with him were two sons, George Henry, 32, and Lonne, 18. "It's quite funny, isn't it, the protocol?" said Lonne, dressed in a double-breasted tuxedo jacket and white silk scarf, sipping orange juice and watching the receiving line. He has grown up with these paintings and knows them well. "They are very well-put-together," he said of the exhibition. "It's marvelously done."

"The modern art, we started collecting together," said the baroness, who married the baron 14 years ago. "Heine," as she likes to call him, "had stopped at German Expressionists. Of course, he had some Impressionists, but we basically shopped together for the modern art."

"What do you think?" inquired the baron after his wife had gone through the show.

"It's very nice," said the baroness, "but I wouldn't have put the Grosz between the two Chagalls."

What is the baron collecting now? "American paintings," he said. "Basically 20th century . . . . I think 20th-century American artists are the best. If you write that people will laugh at me. But so what?"