a few months ago i purchased an asus nvidia 6800xt 128 agp card from newegg and wow, this thing overclocks insane, i have been running it at 421/860 (300/700 stock) at stock voltages. I i tried unlocking the pixil pipelines but no dice... major artifacts, however i was successful at unlocking the vertex shaders to 6 and i am quite impressed with the performance.
I am aware that ATI has a couple cards that are slightly faster at a similar price point but i really don't like ATI for 2 reasons, first, because i think their drivers stink, i've had a few ati cards and while the card performed well, it locked up all the time and that was with a few different motherboards (running xp).
Secondly, while i do have windows xp pro sp2 installed on a small 20 gb partition, i don't remember the last time i actually used it other then for benchmarking (been a couple years). anyway, ati's linux an bsd drivers suck, if you are outside the windows realm, nvidia kicks all sorts of ati butt.
Within the Windows world, my personal experience has been that ATI is slightly faster 65% of the time but 80% of them time nvidia has more solid drivers.
if you are like me and run linux or bsd 99.95% of the time, nvidia is the only way to go, the drivers simply ahinilate the competition. An x800xt could get it's arce handed to it on a silver platter by a nvidia 5600 in linux..... the drivers are that far supperior.