Hilary Duff and Amanda Bynes are about to graduate to the big time. On the heels of ''Agent Cody Banks''' $14 million weekend, Disney Channel staple Duff is gearing up for the May 2 big-screen version of her quirky comedy ''Lizzie McGuire'' and is set to star in Warner Bros. ''A Cinderella Story.'' (We'll see whether she appears in MGM's just-announced Banks sequel.) ''It's her combination of empowerment and vulnerability that's accessible,'' says Disney Channel exec Rich Ross, who agreed to end McGuire's 65-episode run so that Duff could launch her film career. ''She's authentic -- you won't see her on the cover of Maxim.'' Meanwhile, Nickelodeon alum Bynes, who now stars on The WB's ''What I Like About You,'' plays a free-spirited teen in ''What a Girl Wants,'' opening April 4. ''Everyone wanted this part,'' says director Dennie Gordon. ''Britney Spears, Jessica Simpson, Alexis Bledel...but when I met Amanda, I knew she was the one to carry the movie. The boys think she's hot, and the girls think she could be their best friend.''