For a system using two GeForce GTX 570 graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 800 Watt or greater power supply that has a combined +12 Volt continuous current rating of 56 Amps or greater and that has at least four 6-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 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. If your overclock of the i5-2500K is below 4.6 GHz then there shouldn't be any need to increase the +12 Volt continuous current rating to handle this overclock.