mpilchfamily said on 8800GTX power requirements thread :
What you need is a new PSU. The 8800's pull about 15A on there own. They pull 6 of those amps through the PCI-e slot and get just under 5A from each of the 2 PCI-e power connectors. Most systems not including a video card will draw about 15A. So your PSU needs to provide no less then 30A in total on the +12v rails. Now some highe quality 450W PSU will handle this but i recomend a good 500W or better PSU. SLIed 8800s need about 52A on the +12v rails. Now if you plan to or already do some OCing you will want to have a godd buffer of 4A or more on the +12v rails.
Credit goes to mpilchfamily. Thanks to mpilchfamily.
It's pretty simple... you just unplug all of the places it's plugged into (mobo/drives/any existing vid cards), unscrew it from the chassis, screw the new one in, and make sure you reconnect everything.
merwan.. I don't know a UK based link, but the corsair 620HX would be perfect (most times, searching "620HX" at an online vendor will pull it up). If you don't want to spend that much, there are several other decent options to be suggested.
If you are interested, I would suggest going to www.jonnyguru.com and learning a bit about power supplies. He also has a "tiered list" that will clue you in on some of the better power supplies out there for what you need.