I was able to get a Phenom II x4 940 BE to 3.9GHz stable on Air. However it was getting hot and required a large voltage bump. I ended up throttling it back to 3.8 with a smaller voltage bump for long term use.
You may or may not have similar results, depending on chip quality, cooling and motherboard. But I can see a 955 getting close to this without much difficulty.
BEs were designed to be unlocked and overclocked. But you have to be careful and know what you are doing to really push them. Just bear in mind the caveat above.