I was just about to mention barebones kits, but I noticed they rarely have Video cards. The one Kao Linked + 150 dollar video card would be decent medium ish build.
Ok So a quick Tiger Direct Build, No OS, No CDRom, Before Mail In Rebates:
459.94 (before shipping add about 10-15 dollars) Mail in rebates get you 35 back. This Build also comes with Bioshock Infinite and Tomb Raider.
So Overall you're looking at around 425 ish after rebates and shipping. You would have to salvage your old CDrom, or Put windows on a USB jumpstick (This is what I do now days) and your Old Hard Drive (Im assuming its Sata), and as Kao suggested get a OS, or use the same disk from your old pc (Assuming its not a Dell or Prebuilt)
This machine should run stuff mostly on medium to high graphics game dependant. On a my system with a lesser video card (Geforce 460 SE) I can run Batman: Arkum City 1080p with minimal slow down. You could probably run the 2 games that come with @ around medium also, maybe a few settings to low. WoW at mostly high, with draw distance at medium high and get 30+ FPS in 25 man raids with lots of casting, 50-60 outside flying around.
Edit:Also that case comes in white like that sort of thing.
Edited, Mar 14th 2013 3:16am by BeanX