If you're an avid gamer... save yourself from game compatibility issues and check out System Requirements Lab.
System Requirements Lab uses an Active X or Java applet to probe your system's hardware to see if they meet a gam's minimum and recommended system requirements.
To use it just select a game from the drop down menu and click the "Can you Run" button. If you don't have the plug-in already installed, it'll install at this point
After a system check, the site will issue a report on whether or not your system is up to snuff for a particular game. If it comes back negative, the report will list what components failed the check so you can eyeball what hardware you might need to upgrade
Now remember, although your machine might pass the minimum spec requirements, it can still fail the recommended one. And if you really want to enjoy a game, the recommended specs are the ones you want to go with.