The best CPU you can could get for that build is probably an AMD 6300 or the low powered 8 core models (8320e 8370e. I'm not sure which CPU you have right now, but it probably wouldn't be worth upgrading.
AMD currently does not have anything that compares with an i5 4th generation +. Even overclocked at 4.8Ghz, they struggle to keep up.
AMD is releasing their new "Zen" lineup very early next year. If you like AMD, you could wait for that. It's supposed to be comparable to the new Intel lineup in regards to strength per core.