I had a similar setup. A 7900gs, and 420watt psu were replaced with the 8800gt and a 500watt psu. I saw a huge increase in performance in COD4.
I can play at 1280x1024 with all setting on, textures at high, AA/AF off, and get an average of 60fps. I've seen it drop to 30 during some of the large outdoor scenes, but, it was still smooth, IE: no stuttering. The 6600 you have is the bare minimum to play the game so you will see a bigger increase than me, but will have roughly the same fps. Like the above poster said, get another 1GB of ram, it will greatly benefit your system.