I've decided that I'd like to buy an iMac, however my biggest concern is what graphics card to go with. I own an xbox 360 so big titles like Bioshock won't be played on this machine. The only titles that I know I WILL be playing are Diablo III and StarCraft II. With that said, should the ATI 256 be enough? If so, I'd probably go with the 20 inch to save space.