The only problem about running high end games such as Call of Duty 4 on a macbook is the graphics card. The other specs are quite impressive for the size and price, but the graphics card is simply not good enough to run the game at its full potential.
You would have to run the game at low video settings and low resolution, but the game can be run.
Also I am sad to say that you can not upgrade the graphics card since it is intergraded.