The fashionable boutiques on Lafayette Street and in TriBeCa that specialize in groovy postwar furnishings may generate more press and certainly more foot traffic than Mark McDonald's Gansevoort Gallery (72 Gansevoort Street; 633-0555), but the true connoisseurs prefer it that way. So keep it to yourself that this low-key establishment is the city's best source for mid-twentieth-century artifacts. Even the competition concedes the point. "Gansevoort Gallery is the gold standard," says Evan Snyderman, co-owner of R 20th Century Design, a trendy secondhand-furniture store in Williamsburg. "Mark's laid the groundwork for the rest of us." Today, Gansevoort boasts both unique products (a slant-back chair from Frank Lloyd Wright's 1904 Larkin commission is a recent acquisition) and a fan base that regards McDonald's unerring eye and scholarship as definitive. "Mark is the man," says architect-to-the-stars Alan Wanzenberg, who counts Mick Jagger among his clients. "Before he came along, postwar design had a definite yard-sale mentality. Today, thanks to him, there is a curatorial aspect about this field that cannot be denied."