In reply the original question, it's not necessarily the overall wattage of a PSU that's important. It's the 12v rail rating that matters when powering graphics cards. I have seen many lowish wattage PSUs with beefier 12v rails than higher wattage models.
PSUs have at least 3 rails: One delivers 3.3v, one 5v and at least one delivers 12v. Each rail has a limit to the number of amps it can deliver, and all this adds up to the total wattage of the PSU (details of each rail's capability is usually found on a sticker on the side of the PSU, look for +3.3v, +5v, +12v).
For a 8800GT, it's recommended to have a PSU with a 12v railcapable of delivering at least 24amps.
If you're buying on a budget, Google for a "colors 600w psu". They have a 30amp single 12v rail and can be found for about £20 with free delivery from a well known online book store. I can't vouch for these personally but they seem to do the job judging by the reviews.
If you want to spend a bit more and have the assurance of a quality brand, the Corsair HX PSUs are highly recommended. They have multiple 12v rails but are capable of spreading the load between them if one becomes overloaded so in effect they behave like a PSU with a nice & beefy single 12v rail.
