I have an old TNT2 a Geforce2 and a Radeon9000... the ATI is obviously the most powerful card I own is is really good for modern games. I've found all the cards to be very good, especially my TNT2 which I expect would survive a nuclear war...
If I were to get a new card tomorrow, I would get the 6800, because It's had good reviews and it's a good price. Also I find the Nvidia numbering scheme easier to understand.
I would suggest getting the cheapest card that meets your spec.