Can you please list all your current components as well as your total upgrade budget? That will help greatly in making a recommendation.
Running a discreet 7750 and disabling the integrated GPU will yield the best performance, however, since the A6 is only a dual core and not too strong to begin with, I suspect it would bottle neck GPU's like the 7770 and up. For running the 7750 it should be ok for the time being. If you ever decide to run something more substantial like a 7770 or GTX 650ti/boost or better, you'd definitely benefit by upgrading the CPU to something like the Athlon X4 760K, which is essentially an A10-6800K Richland APU with the integrated GPU disabled. It's a very capable 4-core CPU that can run mid-range cards like a GTX 660/ti or 7850/70 without any major issues (and it's not expensive).
So again, a lot depends on your budget and other components as to what would be the best option. As for the power supply, again, depends on your budget. Stick with brands like cooler master, antec, xfx, corsair, seasonic, and try to go for at least 500watts to allow for a little head room for future upgrades.