The additional memory will help your game/apps work more efficiently and not have to use as much virtual memory (when the HDD is used to augment your main memory, a.k.a. RAM). So a performance increase of 10 to 15 percent, when running mutliple apps, is possible.
For your MW2 settings, at the resolution/settings you are using, you are actually stressing your CPU more than your GPU. Adding the additional memory won't speed up your GPU, but will allow the CPU to operate better.
What is the native resolution of your display?