The 660 is a vastly more powerful GPU than the 4770. The 2GB of memory should help with performance quite a bit in itself. Of concern will be your power supply, as the 660 requires a great deal more power. Of no doubt, as pointed out above, the limiter is for sure going to be the CPU, however if you can get the 660 cheap and feel like no other updates (PSU) will be needed, it sure isn't going to hurt performance.
IMO see if you can't find a 750ti to pair with. It should be lower power AND it is still supported for updates. Perf should be quite similar if you get the 2GB model.