6800 GS is not worth 80 dollars more than the 6600Gt, here in canada those are the price difference of the two cards.
Edit: if i was to change the PSU to 480 watt, and in the future i get like a 7800GT and another gig of ram plus maybe a new processor will it still have enuff juice? or is it more safe to not be stingy and pay the 25 bucks for 550Watt