I shall refer you to the Anandtech 'bench' department, whereupon you may plug in the closest possible comparison to your CPU, which I believe was a Phenom 2 X4 920, and compare performance to other cpu's that will fit the board.
The final answer is, it can make some difference, on some games. If it were me, I might go ahead with the decent gpu, and then do mobo/cpu/ram the cycle after that if the performance on your new gpu doesnt feel up to snuff. There's not a WHOLE lot of point long term to adding upgrades to a 'dead' socket/platform. (am3 vs am3+)http://www.anandtech.com/bench/CPU/2
PS: I've mentioned before, and perhaps it will help you in the future. I tend to do my upgrades once a year. I'll do gpu one year, and cpu/mobo/ram the next. Or storage, or what have you. Basically, 500$ a year or so (sometimes more, sometimes less, but 500 or so), and upgrades with a mind for whatever it is to last for at least two years. It actually works really well, although bleeding edge parts quite obviously don't fit in that budget.