Im actually suprised the new iMacs can run cod4 on windows....i keep hearing mixed things about the video card
Those who want high res with max settings won't be happy in anything but a gaming desktop pc, of which the imac is not marketed as one.
But all current games will play at a decent frame rate at a decent quality, which works for many people.
barefeats.com has iMac video card gaming benchmarks along with other benchmarks.