K then:
The Minimum System Requirements for CoD4 says "Nvidia GeForce 6600 or ATI Radeon 9800 Pro." The recommended System Requirements are "Nvidia GeForce 7800 or ATI Radeon X1800."
Since the time-frame you're looking for is "the recent past," that would give the nod to the Nvidia line (which is the better bang for the buck" during that time - and up until about "now.")
Since the price for technology that old & "out-of-date" (yes, GA lifecycle is about 6 months!
), I'd suggest that you get the "Recommended" board, as the price should be pretty good, and therefore gameplay should be what the gamemakers expected people to see.
Hence I'd suggest you get an Nvidia GeForce 7800 - or the best equivalent that can still be found! - like
this one, for instance? ($50 after rebate!)