You'd have to give it a try to find out. Not all CPUs (even of the same model) have the same maximums at a given voltage nor the same voltage tolerance. I'd recommend keeping CPU voltage below 1.4V and you could also give overclocking the CPU/NB frequency a try.
It will vary from chip to chip, but 4.0 to 4.4 Ghz is probably where you will end up.
Make sure you keep an eye on voltage and heat. For some reason AMD lists the max temp on the 4 core FX higher than the 8-core. But if I would still try to sty in the low 60's or better. Max voltage seems to be 1.4-ish. (Although I didn't need that much for the 4100 I overclocked to 4.4 Ghz.)
If your BIOS version allows you to disable APM, disable it. This power-saving feature will cause your CPU to throttle down under full load after a short time. (And thus defeat at least part of your overclock)