For a system using two GeForce GTX 660 (non-Ti) graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 600 Watt or greater system power supply that has a maximum combined +12 Volt continuous current rating of 37 Amps or greater and that has at least two 6-pin PCI Express supplementary power connectors. An exception is that if you have the MSI GeForce GTX 660 HAWK 2 GB, it requires two 6-pin PCI Express supplementary power connectors for each graphics card, so you'll need a total of at least four 6-pin PCI Express supplementary power connectors on the power supply unit.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) is the most critical factor.
Overclocking of the CPU and/or GPU(s) may require an additional increase to the maximum 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 being attempted.