Neither game requires that much GPU power. I would personally get a 750ti and put a decent overclock on it for games like that, but it also depends on the resolution you're playing at. Since you have a set budget, you could browse around amazon and newegg and BHPhoto and look for the fastest GPU on paper that you can afford. If not, you can fish around for used stuff on ebay, and probably score a 780ti for that money as well, but that's a separate and significant risk in and of itself.
In terms of performance from average benchmarking charts:
750ti<gtx950 < rx460 < gtx960
The rest is up to you.