Deerpark is right, games are starting to take advantage of multiple cores. Even if they don't, think logically - if you have one core, and your PC is running Windows and maybe some background apps plus the game, COD4 doesn't have as much CPU power to draw on.
On the other hand, if you have a dual core, one core can be dedicated entirely to running the game, with the other core handling background threads and also the game.
See where I'm coming from?
Eredell: nice card, I agree that you are sorted as regards your video card. Get a Core 2 Duo and 2Gb of RAM to support that and you'll be flying along.