The 6570 is a rather low end card by today's standards. You should be pretty far from bottlenecking it.
For an upgrade, I don't know your model of CPU well, but base it on this. If your Phenom has L3 cache like many do, you will be fine with a 7750 or a 7700. The 7790 or R7 250 is pushing the limits of the CPU though, and I think would have some bottleneck though it would be fairly small I think. Anything stronger would have a significant bottleneck.
i would personally go for an upgraded CPU if you go for a 7790 or higher, since even a $120 six core AMD CPU would be an upgrade and prevent bottleneck.
If you go with a 7700 or lower, I wouldn't bother since there shouldn't be any real bottleneck.