For a system using two nVIDIA reference design 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 68 Amps or greater and that has at least two 6-pin and two 8-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Total Continuous Amperage Available on the +12V Rail(s) is the most important factor.
Overclocking (i.e. factory or manual) of the CPU and/or GPU(s) will require an additional 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.
The Seasonic Platinum Series 1000W (SS-1000XP), with its combined +12 Volt continuous current rating of 83 Amps and with six (6+2)-pin PCI Express supplementary power connectors, is sufficient to power your system configuration with two GeForce GTX 580 in 2-way SLI mode.