plus if your using paeryn's SDL libs you'll probably want to make a HWSURFACE rather than a SWSURFACE.
screen = SDL_SetVideoMode (320, 240, 16, SDL_HWSURFACE);
and you may want to look into the joys of double-buffering:
screen = SDL_SetVideoMode (320, 240, 16, SDL_HWSURFACE|SDL_DOUBLEBUF);