You have made some upgrades to that Compaq maybe you might want to think about a new case. The small case doesn't lend it's self well to upgrading. You may even run into some heat issues. $30 will get you a new case and everything should(key word) transfer over without a problem. Then you can get a full sized card that will give you better performance and a wider selection to choose from.
That being said ATI is the way to go.

99 Contour Sport SE MTX KKM filter, B&M shifter No res, BAT kit Green car silver hood (because silver is faster)