Here is Guru3D's power supply recommendation:
GeForce GTX 750 - On your average system the card requires you to have a 450 Watt power supply unit.
GeForce GTX 750 Ti - On your average system the card requires you to have a 450 Watt power supply unit.
If you are going to overclock GPU or processor, then we do recommend you purchase something with a bit more stamina.
Thermal and Power Specs:
95 C=Maximum GPU Tempurature (in C)
60 W=Graphics Card Power (W)
300 W=Minimum System Power Requirement (W)