Tom's Hardware has another overview article, this time they took a look at the efficiency of AMD's processors. They check out a Sempron, Athlon 64 X2, Athlon X2 BE and Phenom processor.
While the comparison of different Intel processor generations revealed performance per watt improvements of over 400% if you compare the a 3 GHz Pentium 4 630 and a 3 GHz Core 2 Duo E6850, AMD only managed to improve its performance per watt ratio by less than 60% (Phenom quad core compared to the Sempron single core). The performance per watt results will be more impressive if you run intensive workloads; thread-optimized applications will benefit even more from multiple cores. However, we used the SYSmark 2007 Preview suite as it represents the majority of workloads applicable to a wide user base, and also because it does include some idle time, which we believe reflects real user behavior pretty well.