That question again is very dependent on what game you're talking about.
Honestly, unless you're just set on staying with AMD, you really should just hold out until you can afford an upgrade to an Intel CPU. If you are set on sticking with AMD as long as possible, then the FX-83xx CPUs are the ones to look at for an upgrade. AMD is just flat out behind intel in terms of CPU performance in games, and even their best processors need to be clocked significantly higher than Intel's quad core offerings to match them in terms of performance.