Here is what EVGA recommends for systems with it's GTX 580.
Minimum of a 600 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 42 Amps.)
An available 6-pin PCI-E power connector and an available 8 pin PCI-E power connector
For a system using a single GeForce GTX 580 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 8-pin PCI Express supplementary power connectors.