The GTX 750 and GTX 750 Ti are both extremely low power and can work with nearly any power supply. That said, HP's site doesn't list the manufacturer, efficiency ratings etc. of the power supply for the P7-1240... so I have no idea how good it is. I would recommend replacing the power supply with something like one of these:
EVGA 500W 80+B, Corsair CX 500W 80+B (semi-modular).
That said, with the A10-5700 there isn't much point to get a GPU better than the GTX 750 Ti as the A10-5700 isn't capable of supporting much better.
On the AMD side of things, the HD 7750 is basically the only GPU with power draw comparable to the GTX 750/750 Ti, but it's nowhere near as good. If you upgrade your PSU first, then comparable AMD cards to the 750 & 750 Ti are the R7 260/260X/HD 7790 (which falls somewhere in between the 750 and 750 Ti) and the R7 265/HD 7850 which is a bit better than the 750 Ti... however, that creeps up pretty close to the price of the R9 270 which is far better than any of these.
But I wouldn't really recommend a GPU greater than the R7 265 or GTX 750 Ti without upgrading the CPU first.