Well, there is also the next offering from AMD as well. Which could increase performance as much as the Core line did over the Pentium. And you know they won't add anything to 939. Even for mediocre performance gains in the beginning, unless its a flop hardware which AM2 is probably not, I'll always suggest the later.
Then he's stuck with 939 and won't be able to get that new proc without upgrading the motherboard and memory. This way he's at the top of the curve so when AMD goes "he is our next Intel slayer" he can go "Cool, I'll take 1 midrange proc for cheap please," flash a bios if that and be on his way.
I know what you mean
I was just thinking of his budget and the fact that AM2 has no major performance increase over 939, but the "future proofing" of AM2 is a good idea I suppose.
Personally though, until I see a major difference between the two sockets, I`m happily sticking to my 939 and my 4400+ X2.
I`m going to save like a saving person and wait for the next gen of AMD CPU's [ could be quad cores

- Wuff ! ].
Well right now you have the 939 already and you built that a year ago. If you had posed that question I would have told you to stay with 939 and wait for some better offerings from AMD.
On the other hand, if your looking for a good system now I would say go to the new socket type as to give you a good upgrade point in the future.
As for me, I have a Mactel Mini Core Duo and a Palm Treo 650 that work great. When AMD releases dual-proc dual/quad-core FX series chips and Mobos that support them - I'll upgrade to that.