The best CPU your board supports is the FX 83XX series. The entire series uses the same chips with different clocks. Since you will want to overclock anyways, just get whichever 83XX series CPU is the cheapest. The all overclock about the same. I wouldn't spend more than ~$100. You may run into some CPU limits to performance, but you should be able to get above 60 fps with a good GPU. Make sure you get a good cooler.
You may also want to consider waiting to get a new CPU. The next generation of AMD cpus should be released in the next few weeks. That should make higher performing chips available and/or drive down the price of the old chips.