For a system using two GeForce GTX 660 Ti graphics cards in 2-way SLI mode NVIDIA specifies a minimum of a 650 Watt or greater system power supply that has a maximum combined +12 Volt continuous current rating of 38 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 Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most important factor.
Overclocking of the CPU and/or GPU(s) will 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.
The Seasonic M12II-620 Bronze (SS-620GM), with its maximum combined +12 Volt continuous current rating of 48 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is electrically sufficient to power your system configuration with two GeForce GTX 660 Ti graphics cards in 2-way SLI mode. Unfortunately it is deficient by two PCI Express supplementary power connectors so you would need to resort to using a couple of dual 4-pin Molex peripheral to 6-pin PCI-e adapter cables.
If you can get the Seasonic M12II Bronze Series 650W (SS-650AM) model, it has a maximum combined +12 Volt continuous current rating of 53 Amps and with four (6+2)-pin PCI Express supplementary power connectors, is more than sufficient to power your system configuration with two GeForce GTX 660 Ti graphics cards in 2-way SLI mode.
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