I don't understand why the system doesn't devote ALL of it's resources to the task at hand, that's all. If you were using the maximum amount of RAM, CPU, and GPU, wouldn't performance in-game be sensational?
EDIT: I stand corrected about CPU usage. You must start the Task Manager BEFORE you start the game. I did this with F.E.A.R. and it was using 75-80% CPU, but only like 750MB of RAM. Some games don't respond to the Windows button, so you have to CTRL+ALT+DELETE out to check the Task Manager. Still though, it would be nice if all system resources were pegged during gaming.