GTX 750, costs about $100 and can be used with a PSU as low as 220w.
So we don't need a 400W PSU?
No even, Nvidia says 300 watts is fine for a GTX 750 TI. Link.
My HTPC uses a G3258 / GTX 750 TI and it's max power consumption from the wall during 3DMark11 was 114 watts. Since this is draw from the wall, the PC is actually consuming slightly less than that. 300 watts is plenty for a 750 TI
Probably not, that is the manufacturer recommended wattage without knowing what else you have in your system. Any GPU that doesn't have an external plug doesn't draw more than 75w of power. Unless are running a monster CPU or a bunch of other things drawing power you should be fine with a 300w, assuming it's not the bottom of the barrel. Looks like the 750Ti max's out at 68w. If you want to give the rest of your system specs we can run a calculator to make sure.