Update: The Athlon II X4 620
Additional testing with the Socket AM3-based platform reveals lower power consumption with all AM3 processors using the same settings. Originally, we had intended to update the entire article with the results with the new platform, but fate intervened. We do have results with Cinebench R10, which we feel is sufficient to demonstrate performance in the the scenarios we wanted to emphasize (single- and multi-threaded performance).
Below, you'll find the values we measured with the AM2 platform compared to AM3 platform. The two additional cores on the Athlon II X4 620 help it forward in the multi-threaded rendering test, putting it halfway between the Athlon II X2 250 and Phenom II X4 955.
If we look at the total power consumed during a complete run of Cinebench R10, we see there's quite a significant difference between Socket AM2 and the newer AM3-based platform. Although the Phenom II X4 955 BE has higher power consumption at load, total power consumption is actually about the same as the Athlon II X4 620. So, unless you're constrained by procurement cost, the Phenom II X4 955 BE has much to offer compared to the Athlon II X4 620.
One additional note about the platform: we were only able to achieve these numbers after switching the PSU. The lower-wattage Enermax Tomahawk 405W is more efficient at lower loads than the PC Power and Cooling Silencer 750W we used previously. It's also much cheaper (at around $45), although phantom power is higher (2.5W rather than the 1W we were getting previously). This can be easily handled by a using a smart power plug that cuts off power below a certain value. We think this is a much better-balanced platform, since it hits low power consumption and still has the headroom for discrete graphics.