get a 6800nu or GT...you can OC the 6800nu and then unlock 4 pipelines on it, making it a 6800gt, which (i have heard) is the same or better than an x800xl. and it's cheaper...but unlocking isn't guaranteed, so a 6800gt would be safer
plus the 6xxx series have Shader model 3.0, which, unless the computer is upgraded often or you plan to buy a next gen vid card, will be needed for future games.