What kind of Power Supply does my computer needs?

The 8800gtx requires a power supply of a minimum of 400 watts. But to be on the safe side I would look for a psu that has ~500 watts. Make sure it has all the cables you need for the video card your buying and is from a good quality manufacturer such as Corsair. Also make sure that you buy the type of psu for your case type (like atx12v psu goes to a atx case) .