I found this using Google....
Power Supply Requirements
Now here's where we are going to surprise you a little bit more. Ever since the 7800 was rumored on the web immediately the discussion started on how much power it would consume with that huge transistor count. The 7800 GTX, my friends actually uses less power then the 6800 Ultra. NVIDIA claims it uses 10 watts less then the previous high-end model. And that's lovely! I mean we have quite similar core frequency, basically the same memory, 80 million more transistors and yet it's using less power draw. It likely has something to do with the smaller fabrication process (0.11 micron), which normally results in a lower core voltage for the graphics processor.
So NVIDIA is recommending nothing too dramatic in terms of a power supply unit. At the least a 350 Watt PSU with 22 amps available on the 12 Volts rail should be sufficient.
If you decide to become an uber gamer with two 7800 GTX cards in a SLI setup then you do need to look at something with a little reserve though. 500 Watts with 30 amps on the 12 volts rail. Have a look at this OCZ Modstream 520W PSU we recently reviewed. We actually use it in our test rigs, highly recommended as it can peak to 620 Watts.
