I wondered how Nvidia was able to recommend only a 300w power supply for the Gtx 750 ti. I think its not only because it uses less power. Also because it has gpu boost. With gpu boost, it only boosts if you have enough power.
I don't see any problem with putting the 750 ti in any 300w psu computer.
I'll probably put one in my living room computer that has a 300w psu and a a6-3630 cpu. My cpu isnt that great but it will still let me play much more games on that.