Get the ATI X1900XT it's blazzing fast and provides better image quality over the 7900 series, plus you 48 pixel shaders as opposed to 24. I speak from expreience I own the Saphire X1900XT 512MB and FEAR and Prey and all the latest titles runs Oh so smoothly.
Having been making a similar decision myself recently, I have looked at a whole lot of benchmarks, and you can see that there is little to choose between the x1900 xt and the 7900 gtx, they each have a small advantage over te other in some games and not in others. So really it depends on how you would think of upgrading in the future (Sli/Crossfire etc)
edit: *Regrets not looking at the topic right below this before offering opinion*
what a n00b question, the x1900xt will pound the 7900gt, i mean what do u see in the 7900gt to be better than the x1900xt? .. yeah the pixelpipelines, but now pixelpipelines dont really matter, consider if u have more pixelpipelines u can run at a higher resolutions, so if u have a 30" apple monitor then ... nah, just go with x1900xt, its far better