call of duty 4 problem.

i have two questions. im playing this game on the imac with the 20inch screen and i compared it to the graphics for the PC version and the PC version is better. is there a certain screen setting when you go to the graphic option in the game that is the best when playing this game? The other question is when i try to play Call of suty 4 on the macbook, i hear the sound but i cant see anything on the screen. does anyone know what is causing this problem? Thank You