Cotton pick'in hot rods! 
Jeez some of you people have
Smokin' computers and I'm just plain jealous. :rotfl:
Ah what the hey, more power to you.
Snakeeyes heed what the other guys said about the bigger power supply for they are very correct. I've a 7800GS AGP 256mb ( same as yours I think ) and the 7800GS calls for 20 amps down the 12 rail so now we are talking about 500 watts PSU at least. So that probably means that 7900GS 256mb your looking at will demand the same power. If you can afford it ( and have enough room in your computer----------measure first ) get the largest PSU you can handle ( power and money wise ).
@CaptainCox, wish I had that 850 watt PSU but I may not have the room. Whats its dimensions?