If there had been any good socket 939 motherboards that would be somewhat easy to get. I would have said,"stay with s.939!" But the s.939 AMD mobos that is good, is very hard to find.
But AM2 is almost as cheap nowadays. And still have some nice mobos that is pretty easy to find for that socket.
Also, you will allready have DDR 2 RAM if you go AM2 now. Could make the next cpu / mobo upgrade somewhat cheaper, if or when you go Intel Core 2 Duo or whatever it will be ? Next time you upgrade.
I also got the impression you just wanted to go AMD?
As for the "future proofing" it's max 2 years in the gaming rig world. Especially the videocards is short lived. I long for the old days. When a rig would last up to 3 - 4 years, before beeing a dinosaur.
But back to your upgrade. You could get this Core 2 duo rig for a good price:
http://www.newegg.com/Product/Produc...82E16819115004
http://www.newegg.com/Product/Produc...82E16813128044
http://www.newegg.com/Product/Produc...82E16835186134
With the case, the Enermax psu and the Gigabyte 7900GS, the Kingston RAM , the hd and DVD from the previous list + the Belkin antistatic wriststrap, the extra Aerocool fan and the Ceramique paste for a total of 930 bucks.
And I still think that is the best rig for the money... more "future proof" too.
The E6400 with that AC Freezer 7 Pro cooler. Will make it possible to oc the E6400 to about 2.6 - 2.8GHz maybe more? This, as the Core 2 Duo cpu's are cool runnning, and fantastic overclockers. The Gigabyte, 7900GS with the Zalman cooler should overclock well too.
But even at stock speed. The E6400 is one fast cpu!
But now I leave it up to you to decide. And there are others here that may have other upgrade tips that is better than mine.
You're welcome.