Usually websites test how well the latest CPUs overclock, but in the latest article at AnandTech the reporter focused on how well the Phenom II X4 965 can be undervolted to reduce power consumption and lower temperatures. You can check out the results over here.
At a stock core speed of 3.4GHz, we were able to reduce the load core voltage from 1.392V to 1.240V and retain 100% stability with the system. The idle voltage with CnQ enabled dropped from 1.00V to 0.832V. At idle there is a slight difference in power consumption and temperatures, but nothing to get excited about yet.
However, in our CPU load test the system power consumption drops 16% and temperatures 19%. In the gaming test, system power consumption is reduced 11% and temperatures 15%. Of note is that we were able to run the Northbridge speed at 2.4GHz without increasing NB voltages at the 1.240V (1.224 actual) core setting.