What Zing said... I'd go for something from the X1900 range over an nvidia card if I had the cash. Even after the hiccups I've had with getting ATI to play under *nix, I was so impressed by the X1900 that I'm prepared to forgive ATI.
If you're adamant about going with nvidia, then the GTX is better than the GT. I wouldn't bother with the 2 x 6800 option.
If you've got the time and the patience, have a read of
this article over at Tom's Hardware, which covers a range of nvidia cards.