As if the choreography-crazed boy band weren't animated enough, 'N Sync will appear in cartoon form on The Simpsons on Feb. 25. That same night, Ren&#233e Zellweger and rapper Snoop Dogg lend their voices to an ex-hooker and her pimp on the 100th episode of King of the Hill. And Niles fans will get a double dose of David Hyde Pierce on Feb. 27 when, in addition to his regular Tuesday night gig on NBC's Frasier, he moonlights as a self-help guru on Fox's Titus.