For the 939Dual-Sata2, it would depend largely on if you're using the 939 socket or the AM2 slocket, as they have different ddr support. (Built a rig on the eSata2 version, with a 939 Opty 144 massively OC'd...one helluva sweet board.)
Re: 4G and Xp, you're pretty much stuck with 3.5 of it...