A REAL HEART-STOPPER The 36-foot-tall outdoor board, at left, for the Tropicana Las Vegas Hotel, features a Las Vegas showgirl copping some attitude backstage at "The Folies Bergere." McNabb, McNabb, DeSoto, Salter & Co., Las Vegas, created the in-your-face ad to contemporize the city's oldest review, currently in its 35th year. Creative Director Jack Slattery says the billboard, displaying one of the show's leggy dancers in "The Folies" signature outfit, appeals to a much younger, hip audience. "Her expression says, `We're the best. Come check me out,'*" says Mr. Slattery. "The show will keep your heart beating fast."