Burn that copy of Dress for Success: The power suit has been replaced by the ubiquitous twinset. Not just in dress-down offices, but on TV, where Buffy the Vampire Slayer's Sarah Michelle Gellar, Will & Grace's Debra Messing, The Practice's Lara Flynn Boyle, and Ally McBeal's Courtney Thorne-Smith are all part of the sweater set. (So is Meg Ryan, who spent most of You've Got Mail in snappy twosomes.) The starring role for this wardrobe staple usually a camisole or shell paired with a matching cardigan reflects a trend in retail sales. ''Last season, suits just didn't happen,'' says Stacey Kaye, merchandise manager at sweater-set giant Henri Bendel in New York. ''Sweaters replaced the jacket with a twinset and a skirt, you're dressed. It's about separates.'' The look lets Thorne-Smith infuse her character, Georgia, with a bit of her own style. ''It epitomizes Courtney's real life she's soft, she's feminine. She's a girl's girl,'' says the actress' personal stylist, Cary Fetman. ''Ally looks more tailored and tougher.'' Could this ''wool aid'' give Georgia an edge over Ally in the battle for Billy? Keep an eye on the cashmere.