Well, I'd say it'd probably be about the equivalent to the 7800GS. If you were running dual 6800GTs or Ultras, that would be like a single 7800GT or GTX- respectively.
Still, not bad- but nearing time for an upgrade in my opinion. Though I'm still proud of my 6800GT as there really isn't any game it can't play well, I'm feeling its age.
But, I'm holding off on SLi till next year- I want the new tech coming out to have time to mature a bit before I purchase.