http://en.wikipedia.org/wiki/Socket_AM2
Socket AM3
Main article: Socket AM3
Socket AM3 processors are able to run on Socket AM2 and AM2+ motherboards with appropriate bios updates, but not vice-versa. AM3 processors have a new memory controller supporting both DDR2 and DDR3 SDRAM, allowing backwards compatibility with AM2 and AM2+ motherboards. Since AM2 and AM2+ processors lack the new memory controller, they will not work on AM3 motherboards.