Except for the fact that a 6800 is basically a 6800GT with 1 or more pixel pipes that failed, so they sold it as a 6800.
6800 has 12 pixel pipes active, but 16 theoretically available. The 6800GT has 16 pixel pipes active, that is, ALL 16. You can try to "soft-mod" a 6800 to a 6800GT by enabling all pixel pipes, but more than likely, this leads to artifacts, because one or more pixel pipes were tested as not working properly.
I'd say get either the 6600GT, or spend $200 more and get the 6800GT. Don't compromise by getting the 6800.