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 Corsair TX Series TX650 (SKU# 75-001315 / CP-9020038), with its maximum combined +12 Volt continuous current rating of 54 Amps and with two (6+2)-pin PCI Express supplementary power connectors, is way more than electrically sufficient to power your system configuration with two GeForce GTX 660 Ti graphics cards in 2-way SLI mode but it is deficient by two PCI Express supplementary power connectors so you'll have to resort to using a couple of dual 4-pin Molex periphaeral to 6-pin PCI-E adapter cables usually included in the graphics card package.