For a system using a single GeForce GTX 480 graphics card NVIDIA specifies a minimum of a 600 Watt or greater power supply that has a combined +12 Volt continuous current rating of 42 Amps or greater and that has at least one 6-pin and one (6+2)-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Combined Continuous Power/Current Available on the +12V Rail(s) @ 50°C ambient temperature, is the most important factor.
Overclocking of the CPU and/or GPU(s) will require an additional ??? Amp increase to the combined +12 Volt continuous current ratings, recommended above, to meet the increase in power required for the overclock. The additional amount required will depend on the magnitude of the overclock you are trying to achieve.
I've just installed my gtx 480 today - it requires a 6pin and 8pin for the card and you also need a 4pin for the mobo. As my power supply is decent and comes from a good brand, it came with 2x8pin (6+2 pins) and 1x6pin, so 3 power connectors in total. You need this otherwise you won't have enough to power the graphics card and connect into the motherboard.