Changing the PSU isn’t a big deal, they usually fit nicely. You do want to make sure that you have enough physical room for the GPU. Just because a board has a pcie slot doesn’t necessarily mean that the GPU card, which is usually the size of a small Volkswagon, will fit in without crushing other components.
730s are seriously crippled, IMO, and 740s are slightly more power-hungry for less bang. If you want specs (from which I’m talking) go to http://geforce.com.
I try to balance number of cores/clock speed/RAM type/wattage, others may suggest another metric to compare cards.
In terms of value, the 750 is rated pretty high (better than 750Ti).
With respect to power consumption, it is first generation Maxwell so performance per watt will be much better than Kepler cards (eg 650 or 740) but not as good as 750Ti, which is also first generation Maxwell. I believe that max VRAM will be 2 GB which is not great but better than 1 GB; I’d suggest you go for a 750Ti if you cannot find a 750 with 2 GB VRAM or if you find a 2 GB version and there is little cost difference between the two.
Note that recommended power supply for both 750 and 750Ti is 300 watts per Nvidia specifications (I was expecting a little higher).