Your answer would depend on how demanding the game you want to play will be. I would say an RX 560 2GB would be the best choice for the least amount of money. If you want to know a bottleneck for your system. You can use game-debates can i run it at http://www.game-debate.com/can-I-run/
I believe that the cpu, gpu, ram, and gaming companies pay them to tell you through their 'recommended lists.' I have no proof, but my meager gtx 1050 in my laptop will run final fantasy 14 at full resolution at the highest settings and get 90-120fps. It says on that website that it is botttlenecking, but I don't ever see a hiccup.
Best of luck!