For
this game Nvidia is theoretically better, since it was optimized for nvidia cards. Note the startup logo. In general I'm happy with my x800 xt platinum, and it runs the game pretty well with most of the settings maxed out at 1600x1200, and some more conservative shadow settings.
I'm sure someone without a 1600x1200 lock in lcd could run the game smoothly on this card at lower resolutions.
Likewise, the 6800 ultra should fly. For mid range cards you'll have to turn down some settings, but the game is pretty flexible as to what it will run on. I have friends who run it on 80.00 cards (5600 ultra ?) just fine.
If you're talking gaming in general the top end ATI cards tend to outperform the nvidia cards in most games. Not eq2 and not doom 3, but most other games. Unless you can go SLI with two gts, in which case your machine will be crazy fast(empirical measurement
Nvidia performance will continue to improve with new driver releases, so who knows what the future holds ? ATI is starting to become more competitive in that area as well.
Just my chunk of gold.