We already compared performance per watt over four generations of Intel CPUs. Now we compare AMD's Sempron, Athlon and Phenom CPUs to see if efficiency has increased over time on the AMD side.
http://www.tomshardware.com/2008/01/30/processor_efficiency_compared/index.html
http://www.tomshardware.com/2008/01/30/processor_efficiency_compared/index.html