It doesn't matter at all. The 6800XT is a total rip-off, the plain 6800 will kill a 6800XT.
Go for the 6800, since it's a better card.
I agree the 6800XT 512MB extreme is a ripoff, but it is a 350/1000 clocked card and would come close to reference 6800 performance. If it had been the 256MB 6800XT extreme xxx edition, it would be clocked 420/1100 and it would beat a plain 6800 in most cases. Have a look.
Moral of the story, always check the clocks, pipes, mem interface, etc. There are so many whacky variations out there including both of those ones digit-life has in their charts. But in no way would I recommend either card without knowing prices. My gut feeling are both are a ripoff compared to what else is out there.
Where are you searching(local, online, what country?), what are your system specs(CPU speed and AGP or PCI-e?), and what are the prices of those two cards?
Neither have caught my attention of being good buys online in the USA, your pricing/availability could be way different. Of the two cards, you would have to make sure of the clock speeds and memory interface: Avoid 128-bit 6800's. A 350/1000 clocked 6800XT 256-bit would be a bit behind a reference 350/700 6800 256-bit(edumacated guess :tongue: ). But if the 6800 is a 325/600 like some of Dell's or others floating around, they would likely be very close in performance and I'd think there has got to be better choices for you.