GeForce GTX 750 Ti doesn't even support SLI mode so you don't even need to worry about that.
System Power Supply Requirements for a single GeForce GTX 750 Ti
• NVIDIA specifies a minimum of a 300 Watt or greater system power supply. (Minimum system power requirement based on a PC configured with an Intel Core i7 3.2GHz 130 Watt TDP processor.)
• the system power supply must also have a maximum combined +12 Volt continuous current rating of 20 Amps or greater
• the system power supply should also have at least one 75-Watt 6-pin PCI Express supplementary power connector for some of the non-reference design cards (e.g. ASUS GTX 750 Ti OC 2 GB [GTX750TI-OC-2GD5], EVGA GeForce GTX 750 Ti FTW w/ EVGA ACX Cooling [02G-P4-3757-KR]).
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.