First off, they are two totally different sockets(AM2 and Socket939) so you have to get the correct motherboard. Next, I beleive the AM2 socket allows for DDR2 RAM, but I have not really gotten myself up on this just yet... Also, at least with the previous 939 models, there are different versions of the processor. Some have SSE3, some don't. Some use a different process, etc etc. However, being the same price, they probably are the same.
Really, the question is do you want the newest and best motherboard/upgrade path?