The 6800 series still beats the 6600 series. However the XT is a budget card and the GT is not.
However, some people said you can mod the 6800xt to get the extra pipelines and make it run just like a 6800gt, but I can't be sure. It might be better to get the 6600gt for future resale value, as budget cards don't sell well. But then, you could buy the XT and maybe mod it, although it still may be a small measure better then the 6600gt.
And then again, a 6800GT may not be that much price difference from the XT, being that the 7800 cards are out and pushing prices down. Save for the GT if you can.
That's my take, but some here know more about nvidia cards then I do.