I can't speak for the X800XL, but I just bought a 6800GT and it's doing even better than I anticipated.
I run at 1600x1200 with high settings (dynamic shadows and dynamic lighting are set to medium). Without 4x AA I get between 50 and 60 fps. When I turn on 4x AA I get 45 - 55 fps with an occasional drop into the high 30's.
I run an exhaust fan below the GPU and I never feel it blowing hot air, so I presume the card is running cool. Of course, my case has two front intakes and a total of 3 exhausts, so it's a tad loud, but I certainly don't have heat problems!
Anywho, I'm quite happy with my purchase. It was more expensive than the 800XL, but whatever.