Here’s an interesting site that’s useful in a pinch for determining whether or not a certain game will be able to run on your hardware: Can You Run It.
Basically, you select the game from the dropdown and hit the “can you run it” button. This will work in either IE or Firefox (sorry Opera folks…it’s a no-go). It then gives you a breakdown of which pieces of hardware are holding you back (CPU, RAM, video card, etc) according to the game’s minimum requirements and recommended requirements.