first of all, there are heaps of X800 verses 6800 threads in this forum, so do a search next time.
ok my personal preference is that the 6800 GT is the best one to get out of all, you can overclock it heaps, like was said, to more than the ultra.
I liked the 9800's over the 5950's, but the 6800's have really taken off, and beat the X800's in nearly all areas.
although when AA and AF are turned on, the X800's do win sometimes, but when they do, it's not by much. however when the 6800's win, they win by a big margin. then to top it off is the better image quality from Smartshader 3.0
when you buy a 6800, it doesn't look that great, but you could always replace that cooler, to say an arctic Cooler Nvidia cooler 5, which looks better, and works better and quieter than the stock cooler.