Your choice of Graphics Card ( a great choice I must say) shows that you are after a budget build. Best budget CPU I can suggest for that GPU at the moment is Ryzen 3 1200.
It is crazy cheap at about 100$ and will get you gaming at 60fps 1080p med to high settings which is what that graphics card can easily handle. You can also play e-sport like overwatch and cs:go at high fps (100fps+) with that CPU and GPU combo.