Seeing as I've owned all three of those cards, I can tell you that the x800 pro is superior to both Nvidia solutions. At first I had the 6600gt, good performance, quiet MSI cooling solution but I upgraded to a 6800 unlockable for playable frames with high AA/AF. I bought a BFG 6800 and unlocked it via rivatuner and that was a great solution for a few months but then I saw an x800 pro refurbished direct from ATI for $185 shipped. The 6800 was fine but it was big for my SFF and ran hotter than the x800's. Also the x800 pro has better performance than any of the Nvidia's I've owned when playing high settings in new games: FEAR, BF2, etc. If you can spare some cash, I'd go for the x800xl but the pro is fine.
if you can spare $39 more you should get the Asus AX800Pro VIVO.
http://www.newegg.com/Product/Product.a ... 6814121519 Unlock the extra 4 pipes and have x800xt pe for $239.
I got my Asus AX800Pro VIVO on 9/9 and its a x800xt pe now with no problem (16 pipes 520mhz core, 1120mhz memory). You don't even have to modify the hardware at all its a softmod.
Sempron 64 3100 @ 2430mhz | 512x2 Coarsair XMS DDR400 cl2-3-3-6 1T | DFI LanParty UT NF3 250Gb x800pro -> softmodded to x800xt pe | Enermax 535W EG565P-VE