Any nvidia card will work on any ATI or Via chipset muchlike any ATI card will work on any nvidia or via chipset...generally speaking a video card won't have much dependency on a chipset, however that said, the nforce3 chipset has some serious issues with nvidia cards which includes stuttering in games and crippled overclocking
You can't really compare ATI and nvidia anymore as it's muchlike comparing AMD and Intel, apples and oranges etc. Both will excel in different areas and both will run into specific issues
ATI has choosen to limit the actual pixel pipelines and clock frequencies in the x1900XT and increase pixel operations per cycle whereas nvidia has choosen to continue to buff up pipeline count and clock speeds while maintaining a fixed pixel operation count.
IMO, this translates into ATI having more eye candy capability whereas nvidia has more raw power. So basically, if you want to turn on HDR and shadows and start bumping up AA and AF, or if you like playing at high resolutions, the ATI card should do better. If you just like cranking up the settings and don't care about eye candy and simply want a higher framerate, you want the nvidia card
If you choose to go the ATI route, better to save your money and just buy an XT rather than an XTX as the difference is minimal and nothing a bit of overclocking can't fix, and if you want nvidia and want to save a bit of money while sacrificing a bit of performance, you want the 7800GT