COD4 on the Mac
I've played COD4 on Vista Home Premium via BootCamp on my 2.4 Core2Duo MBP. Ran great.
I bought the Mac version a few months later, hoping it would run smoother/faster/etc. Not so much. At identical resolutions, it ran better on the Vista side of the house, with the only downside being the noise generated by the constantly running fans. I don't mind poking fun at my PC-using peers that bought Dell's fully loaded XPS laptops around the same time I bought my MBP and my game experience rating is a 4.8 (and theirs is 4.9, but they paid $1500 more for their laptop than I paid for mine).
If you've set up BootCamp and can run other games on Windows/Vista, you'll probably have success with COD4. The only computers I've seen that were unable to run COD4 have been budget PC laptops with integrated graphics.
If it doesn't work, there's always ebay. Someone will buy it from you.