Let's see the default voltage settings for the six processors and their respective p-states in K10Stat.
Athlon X2 7750
Athlon X2 7850
Athlon II X2 250
Phenom II X3 710
Phenom II X4 945
Phenom II X4 955
The Phenom II X3 710 is using the same core voltage below 1.9 GHz, regardless of clock rate. In addition, why does it need a boost in northbridge voltage at the lowest p-state? We suspect this is simply not needed, since the northbridge doesn't change clock rates (1.6 GHz).
Note: additional testing on an updated platform using both Socket AM2 and AM3 motherboards reveals that you don't have much room to undervolt the northbridge on an AM3 motherboard. In our discussion with AMD leading up to this story, the company stated that the values are chosen with good reason, and undergo a thorough testing process. We believe the settings were optimized for Socket AM3-based platform. With only core voltages with which to play, we saw a very small drop in idle power consumption (8W) via undervolting. That is in addition to a difference of 10 watts, which means power consumption on the AM3 platform is roughly the same as an undervolted AM2/AM2+ platform.
AMD is also quick to point out that running Socket AM3 processors on an AM2 motherboard (single power plane) is not officially supported. It is still possible of course, and we encountered no problems whatsoever during testing (even with forced Overdrive settings for the Phenom II X4 955 BE). However, that doesn't mean it's an optimal configuration. The power measurement tests we ran confirm this. All else being equal, you should see a Socket AM3 platform consume less power than an AM2/AM2+ platform with the same AM3 processor installed. In retrospect, AMD explicitly stated this when it first launched its Socket AM3 processors.
One final note about these voltages. You can see from the screen shots that the option for changing core and northbridge voltages is available with the Phenom II X4 945, but not for the Athlon X2 7750 or Phenom II X3 710. That's not due to differences in the processor, but rather the motherboard used. Core voltage selection in K10Stat was not available on the Biostar TA790GX 128M, but it was a viable option on the Gigabyte MA-790GP-DS4H. This relates back to the number of power planes implemented by the board vendor. The TA790GX 128M is not a dual power plane motherboard, so you can’t change the core and northbridge voltage independently of each other.