You would see a difference with the 6800GT yes, but you said up there the 6800
Video cards made by Geforce work like this:
Standard, GT, and then Ultra (best)
If you got the GT (would cost more than the standard) you would get much better performance than the ATI card with no overclock.
The standard one though if you don't want to overclock would lose out from the ATI X800XL which would beat it in most games, with the frames per second of course being higher, unless you overclocked, in which case, it could match it!
So its up to you. It seems you have made your choice with ATi though since its cheaper, and will out perform in some places, so yeah.
I hope this helps