If you already have components for 939 board, there's no real reason to change. AM2 CPUs do have a small advantage - better upgradeability - and they support hardware virtualization, if you're into that. But DDR3 is already on its way...
Yes, it is. 939 is going to be an AGP board with DDR..Am2 is worth the extra money.
AM2 is more then likely going to be a PCI E board, with DDR2, and the newer models of processors. Also, look at 775 boards. They are the newest intel sockets.