Not to mention the possible need for Registered memory, which is pretty pricey and has a notable performance hit vs. using un-registered sticks. You'll have to factor in these costs as well, or choose wisely with this in consideration.
I also agree that 939 is worth waiting for, then just order up the entire base components together. An Abit/Asus AV8/A8V mobo and a 3500+ should be a big jump for you and think you'll be much happier in the long run. This will take some money ($500 sound like a good forecast for both?), so if you are strapped and don't think you can get this down the road, I really don't know what to say.
I believe only the Socket 940's need registered DDR, and since he mentioned an A64 2800 proc. he wouldn't be able to use it with a Socket 940.
Another vote for the Socket 939 setup. It may cost a little more, but to me it's worth it. Not only can you use a 939 A64 and A64-FX proc., but you also don't need the dreaded registered ram. So with a 939 setup it will surely be the best as far as upgradability goes. I plan to do this as my next upgrade in a month or so.
EDIT: You can also try overclocking as athlon11
mentioned. Spending less money for the cooling hardware possibly needed.