The 750 Ti isn't built to handle more than what the PCI slot can give it (66w).
There's an cryptomining article that talks about the TDP of the 750 Ti (
How to Increase the GeForce GTX 750 Ti Power Target Limit), and they're setting the maximum power limit to 65.5w. I wouldn't think you would even pull that much power with a factory superclocked card. If you read through the article, it seems like NVIDIA put the stock power limit at just under 40w in their own BIOS.
Bottom line: the 750 Ti is a super light consumer, and the maximum power you will likely pull from it, without a custom BIOS, is probably going to be ~60w. Your typical 450w 80+ Bronze PSU will do just fine. A 300w PSU will be okay-ish, depending on which one you use.
EDIT: if you're using an old PSU, you should be okay with a 300-350w unit. If you're buying new, I'd stick with 400w at the low end, unless you have a really low TDP CPU. If you OC, then 400-450w is likely your best bet.