I played CoD MW and MW 2 on my 17" MBP with a lowered resolution (1280 x 800 or 1440 x 900) and it looked more than fine for me. I also played CoD on my 20" iMac with a lowered resolution (1280 x 800) and it looked really okay.
It's kind of like watching video on your screen in full-screen. The more upscaling it has to do, the worse the video quality seems to get.
Anything above 1024x768 will look good on your screen.