help with SDL and frame rate

Hi all, If someone has a pointer to where I can find more information about maximizing framerates for 2D games under SDL, I''d really appreciate it. Otherwise, here is my problem. My game is a 2D RPG, where the hero is always in the middle of the screen and the background scrolls. Thus, it seems I need to blit the entire background each time the character moves. When running 320x240, this is fine and I can get 300 fps, but if I kick it up to 640x480, it drops all the way down to 45 fps. Each game cycle is only calling two blits - the background and the hero sprite. Anway, any advice is appreciated! Ty