From throwing sand to throwing over-the-top birthday parties, another Michael -- Capponi -- is having his splashy, flashy birthday bash at La Cote on Sunday. According to Capponi's spokeswoman Lena Makurath, ''The festivities begin at 1 p.m. Sunday afternoon and will be 1960s St. Tropez theme so bring out your inner Brigitte Bardot -- bikini, floppy hats, strappy sandals -- because costumes are mandatory.'' So is an invitation. As for what the men can wear . . . Speedos, we guess?