I think you would be surprised with how good you could game on a 7750 on that rig.
It won't be a world beater, but it will play many game and 'playable' rates, depending on the resolution of your monitor.
I upgraded an old celeron system with a similar motherboard a while back. Put a 4770 in it, and it does just fine. a 7750 is about the same performance, a good price and a lot more power efficient.
Here is the link to my old thread:http://www.tomshardware.com/forum/283580-28-upgrade-build