The ASRock M3A785GMH/128M mobo might be a good bet for the Athlon II X3 CPUs.
I'm using an Athlon II X3 435 on one and it works quite well.
I haven't really tried overclocking very much yet, although I did play with the settings and found it to be quite easy to overclock. (You can do it within Windows using a utility that ASRock supplies.)
I was able to unlock the 4th core on my CPU.
This mobo supports AMD/ATI's Hybrid CrossfireX, so it's probably most cost-effective to go with an ATI card if you decide to install something with better graphics performance than the onboard Radeon HD 4200.