The power supply recommendations on video cards are based on the power supply ratings provided by the manufacturers of the $5 power supplies bundled with computer cases. Various white-box, stamped-steel units
rated, and are liable to release the magic smoke or even catch fire if they're asked to deliver anything near their rated load for more than a few seconds. To get around this issue, video card manufacturers routinely recommend a power supply with roughly five times the required capacity.
A high-quality power supply actually will
deliver rated capacity for extended periods of time. Conveniently, pretty much all cheap power supplies are also loud, so anything suitable for an SPCR computer is going to be pretty capable. If you've got a proper power supply, then you'll be pleased to know that, in normal usage, an 8800 GT won't draw more than 80 watts, and even the mighty 8800 Ultra can only burn through 140 watts. This might go up for Folding, but I doubt it'll even hit the TDP mentioned in an earlier post. Most 8800 users, even those with an Ultra and a quad-core, would be just fine with a 300-watt PSU. Even a money-no-object overclocked quad core/SLI insane computer will struggle to pull 600 watts.