AM3 processors can be used on AM2 boards because they still have a DDR2 memory controller on chip in addition to a DDR3 memory controller. AM3+ processors are entirely different and will not work on anything except an AM3+ motherboard; among the changes with AM3+ (FX) processors is that they do not have a DDR2 memory controller. They will not work on an AM2 board.
There have, however, been AM3 boards that are capable of running AM3+ processors but only an AM3 board can do this
and even then the support is hit and miss if successful at all.http://en.wikipedia.org/wiki/AM3%2B#Socket_AM3.2B