VGA Charts - Call of Duty 4
This is up to date as of the last major card releases by both ATI and Nvidia. Should help to eliminate anymore graphics card questions.
IMO if you have and LCD monitor, and most of us do, you need to be playing COD4 at your monitors native resolution. That means you need to buy a card capable of averaging at least 45FPS on this chart at the native reolution of your monitor. The reason I say at least 45FPS is because this test was done on "All Ghillied Up". That level is the most graphics intensive level in the game and your FPS in other levels, and most importantly in multi player, will be considerably higher in most cases.
Also multiplayer FPS needs to be above 65 on average for the best possible experience. For me I have to disable Anti Aliasing to achieve this but I can't tell the difference since I play on my 22in Samsung @ 1680x1050 which is pretty much crystal clear no matter what. I would also suggest that you manually set the aspect ration of your monitor in the options menu since I have noticed a slight issue with hit detection when I leave it set to Auto.
Last thing. I have an XFX 8800GTS 512MB video card. After testing about 50 different configurations in the control panel I have would suggest that you leave all settings at the default level in the CP for best performance. That is unless you have a better performing card in which case you will have to test out if you can tweak the options. I will be getting either a Radeon 4870 or a 4870X2 in the next few months and i will post some info about that after I test it out.