Overclocking the Athlon II X2 250 we had pretty good success ending up just short of 4GHz. It wasn't quite stable but 3.8GHz was smooth sailing.

Our Phenom II X2 550 didn't overclock quite as good being more stable at 3.8GHz and generally stable at 3.7GHz.

Our power consumption was tested with the Radeon 4850. Idle was taken 10 minutes idle into the Windows desktop and load using Cinebench 10. We don't see much of a difference at idle between the two processors but more so at load the extra cache of the Phenom II starts to kick in.