the x850xt pe was ati's last generation flagship card. i don't know much about the agp versions.. but i know the pci express versions of x850xt pe is faster than the 6800ultra but slower than the 7800gt. the 7800gs i presume is slower than the 7800gt, so i presume that the preformance should be close..
i found some benchmarks here.. but they're using a 7800gs pci-express... i thought they only made it for agp..?!?!?!?!
anyways.. in those tests the 7800gs in most cases made quick work of the x850xt pe. there might be a difference in agp versions.. i donno..
anyways, depending on how much you're willing to spend and how much those cards cost, you really can't go wrong with either of them. if they cost hte same, then go for the 7800gs.