Depending on your budget and how conservatively rated that PSU is you could get either an HD 4670 or an HD 4850. You should try reading the label on the side of your PSU and provide us with the info so we can tell you more accurately what you can and can't use.
MSI R4670-2D512 Radeon HD 4670 512MB 128-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Retail
Apparently the combined 12V rail looks like it's rated at 30A, usually I'd say to look into another PSU, but if the PSU is fairly recent you could probably do away by just getting a graphics card. For your budget the obvious choice is the HD 4850.