Because of the extensive use of overdraw (smoothing, blur, blooming) in this game, to enable AA on top of all this will require a top-end GPU and a 3GHz processor to achieve decent framerates. Its all about fillrate and pixel shader performance. Apart from these overdraw effects the COD2 engine is pretty basic, so on cheaper computer systems I would tend to turn off smoke smoothing, Anisotropic filtering and definetly Anti-aliasing completely. If you got a 3GHz, 1GB RAM and a nice non-integrated soundcard it should run as good as the developers intended to on most x800/x1000/6x00/7x00 parts.

I play it at 1024x768@75Hz, 16x AF, 2x AA with all settings maxxed and I achieve 25 to 45fps throughout even the busiest battles. Even peeps with XBox360s will have nasty drops in performance with this game, so dont concern yourself too much.