I second that decision if you're going the AM2 route and plan on overclocking. However, I don't really think there's a whole lot to gain going from 939 to AM2, aside from DDR2 RAM. I can't really tell a difference between my brother's AM2 rig (with an X2 4600 @ ~2.8GHz) and mine except in multi-threaded applications, but 939 has dual-cores as well.
I personally think you'll be fine getting an Opteron 165/170 (assuming you OC it) and sticking with the 939 rig, unless you plan on doing more upgrades to your RAM/CPU before AM2 is phased out. Just my 2 cents.