Well most 7750's don't need a power supply connection, while the 7770's do, that means they need more power and your PSU has to have a 6 pin PCIe power available.
This will give you a rough idea of the differences in the same games, same system, just different cards. http://www.anandtech.com/bench/Product/535?vs=536
With that said, I have the 7750, got it on black friday as the place was sold out of the 7770's and I was going up from a 5450, so anything was an improvement. I can play black ops 2 at 60fps with just about all the details turned on full at 1280x1024 resolution. I play battlefield play 4 free at max at the same resolution.