The 9950 is an AM2+ socket, an AM2+ mainboard can definitely handle an AM2 chip, but I don't know if the opposite is true... According to http://en.wikipedia.org/wiki/Socket_AM2%2B it should be fine...

I'd definitely pop the old chip back in and see if it still posts, if so, I'd update bios and all, one poster above says that you've done all this but I don't see that stated anywhere... so I'm blind, or something. Still doesn't work after verifying the MB is compatible, then you have a DOA chip.