I'd pick up a 6800GT over an X800XL, if they're the same price. My thinking (all AFAIK, not from experience):
Speed: GT nudges the XL.
IQ: GT allows for extra effects in some games (due to SM3 and HDR). OTOH, it may have that texture shimmering issue? Edit: I may be
wrong about the shimmer, or it may be very game-specific.
Power draw: about even.
Noise, warranty and bundled games and features (VIVO, component out): depends on the individual cards.
Drivers: Probably a wash in Windows. GT for Linux.
Compatability: apparently 6800s have some trouble with nF3 MBs. This is a pretty specific case, tho.
Price: this is probably the deciding factor, and it obviously varies.
Depending on the IQ and benchmarks in the games you want to play, I'd see spending $20-30 (so, around 10%) more for a GT.