AMD CPU Efficiency Compared
 

A 100 MHz difference between the Athlon X2 BE-2400 and the Athlon 64 X2 4600+ is small enough to have little impact on the overall results. What mattered to us was to expose the advances AMD made when switching from one product generation to the next. Let's look at how AMD's performance and performance per watt ratio improved over time. We'll start with the single core 90 nm Sempron, look at the dual core 90 nm Athlon 64 X2, continue with the dual core 65 nm Athlon X2, and conclude with AMD's latest product, the 65 nm Phenom quad core processor.

http://www.tomshardware.com/2008/01/...ency_compared/