You don't want to be overclocking the FX too much unless you somehow own a nuclear power plant...http://www.tomshardware.com/reviews/bulldozer-efficiency-overclock-undervolt,3083-19.html
yes the FX is not that power efficient , but the 4100 is acceptable . The difference is about the same as the cost of running a light bulb . A small light bulb .
And interestingly according to Anandtech the Opteron editions of BD handle more threads and do more work for less power than the Xeons .