AF actually doesn't affect your FPS nearly as much as AA does, you're more than welcome to keep your AF up, as I do the same without loose but a few FPS, AA is off though =(. The only time I lost a major amount of fps was on the last level, when I was on that damn volcano thing... Man, there was lava flyin' everywhere and my game just got sloppy as hell. Like I seriously think it went down to 20fps. Beedubaya, I think it's just that your AA is up so high. Reviews on the xt card say "Now you can have no fear of playing some games with AAx2 on!" Which means that would totally cut farcry out of the no fear idea hehe... if you put AAx2 I bet you'll be running 30 fps at the lowest on the level you're at in farcry.
*edit* on a side note, if you have enough ram, running programs in the background are fine... typically it will all be held in the ram without anything being written to the hard drive. If the game you're playing wants more ram, everything that needs to will be written onto the hard drive (background programs). And that will slow you down.