What games are you playing?
For PCI-E at least, the X800XL far outperforms the 6800GT in Half-Life2 and it's lead is cut a bit in Battlefield2. In DOOM3, however, the 6800GT and X800XL are about equal. As you increase in resolution and anti-aliasing, the X800XL will extend its lead over the 6800GT.
The real advantage of the 6800GT are SLI, which is probably not a very good way to go, and SM3 (as far as I know, that's all there is). I don't find either of those terribly important.