Looking For a New Tower With Windows 7 and Gaming Graphics Card

I currently have a Windows XP computer with an Intel Pentium 4 Processor and 321 Gigabytes of RAM. (Suckish, I know.) It is a Dell Dimension 4550. It's also nine years old. That's pretty much all I know about it.

I was looking to upgrade to Windows 7 and I'm a hardcore gamer so I need one of the best graphics cards. I'm only looking to replace the tower and I was going to reuse the speakers, monitor, mouse, and keyboard.

I'm looking for something in the $200 - $400 price range. I want only the tower and no monitor or anything else. If you find something higher in price that I might like, it's okay to show me anyway.