I am sorry, but I have to throw an opinion in here. I would not count on AM3 CPUs to work on a AM2 board. My supports for my opinion is...
The overall lack over interest in AM2's...not a whole lot of PC manfacturers use them a lot and there hasn't been a huge fanfare from the building/over clocking comunity over them either.
Then you add the fact that any time a major change to a CPU came along in the past there was ussually (if not always) a new socket developed.
I am fairly sure that there has never been a public anouncemnt of AM3's working on AM2 boards, just a statement that was suppoedly made to PC manufacturers. This behind closed doors statement has been the only proof I have ever seen to support the upgradability.
The socket AM2 has been out for quite a while now...by the time AM3 comes out AM2 will probally be severely out dated as far as hard ware goes. I have been expecting to hear that AM3 will be scrapped all together,
And last and probally most important...if you could upgrade to an AM3 CPU then AMD would make it very, very, very, very well known in hopes to sell more AM2 stuff.
Go for the the cheaper memory and mother boards that come with the 939.