All Phenom and Phenom IIs support DDR2, and frankly, the performance difference between DDR2 and DDR3 isn't that big. There are good boards that work with DDR2 and the same boards (with different memory slots) that work with DDR3. If you don't want to go through the hassle of selling off your DDR2 and buying DDR3, just get a board that supports DDR2, they're still making current, up-to-date boards with socket AM2+ (for instance, the 785G boards).
Rifterut is right, though. I remember buying 2x2GB DDR2 for ~$40, and looking on newegg, the same RAM is now ~$90. You could come close to breaking even by selling your DDR2 and purchasing new DDR3, which would be more "future-proof", though no system or components will ever be.
I'm just waiting on DDR3 2x4GB prices to drop to a level that's close to affordable...