DDR2 is expensive for price/performance.
Now, all depends of the processor that you want install. I have a X4 955 on a mobo socket AM2+/AM3, and I can tell you that your will get high temps on your processor with an AM3 processor in a mobo socket AM2+/AM3. IMO, is better get a mobo AM3 and DDR3, you can build a solid right without spend much money.
Even, you don't need 8GB or 12GB of RAM, more when that RAM is DDR2. You can find 4GB DDR3 1333 at $114.99 on the egg.