For a system using two GeForce GTX 590 graphics cards in 2-way Quad SLI mode NVIDIA specifies a minimum of a 1000 Watt or greater power supply that has a combined +12 Volt continuous current rating of 80 Amps or greater and that has at least four 8-pin PCI Express supplementary power connectors.
For a system using two GeForce GTX 580 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 900 Watt or greater power supply that has a combined +12 Volt continuous current rating of 62 Amps or greater and that has at least two 6-pin and two 8-pin PCI Express supplementary power connectors.