At a glance
7770 has bandwidth of 72.0 GB/s
GTX650 has a bandwidth of 80.0 GB/s unless it is the Ti version @ 86.4 GB/s
both entry level cards for Med/High gaming
might want to look at the new 700's series from Nvidia instead of the 600's
I'd get the GTX 650 2GB, the VRAM is important, then overclock it to GTX 660 speeds. I have AMD myself so I am not a fanboy, although I prefer AMD normally.
Guys, nothing wrong with posting your suggestions, but at least, let's try to post something accurate.
1) The specs on paper matter little, the HD7770 is a good 30%-35% faster than the GTX650.
2) Overclock a GTX650 to GTX660 speed? You mean clock, yes doable, but why, it is still underpowered. Overclock it to GTX660 performance? That, I want to see. The GTX660 should be close to double the performance of the GTX650, meaning you need a 100% overclock. Oh, the 2GB RAM in a GTX650 is useless, the card lacks the firepower to use it.
OP, no contest at all, the HD7770 clearly, but as others suggested, if you are willing to play the rebate game, the HD7790 is barely more expensive, and a good 30%-35% faster.