Depends on how far you want to push voltage. The official limit is 1.35v, and AMD says that you can push to 1.45v but suffer degregation. From what I've heard 1.4v is a good spot to max out at.
You won't know what your max frequency will be until you try. Start at 3.8ghz (38 on the multiplier) and push it up in .1GHz increments until your unstable (use stress testers like OCCT to find instability).