The X800XL though about the same specification, the 6800GT will probably perform a bit better, though not that noticeable probably in most games.
it depends really who you like best as a card manufacturer, though I don't like the drivers in ATI cards, and don't think they have some power Nvidia has.
I expect that they'll use about the same amount of power though.
Nvidia have definietly won me over in the years I've kept with ATI, as I have a Radeon 9800Pro now, i'll probably go for a Geforce 6600GT, so its up to you
I hope this helps