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 Compare Prices on 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.
Most of you probably agree when I say that Phenom is still at the beginning of its career. AMD has had difficulties finalizing the product, and I'm sure there have been manufacturing and yield issues as well. However, looking at the power consumption results and the sophisticated power management used to allow each core to be controlled separately, we are sure that future steppings of Phenom will introduce more power savings and better efficiency. Support for this expectation can be found if you compare the results of the 90 nm Athlon 64 X2 4600 (2.4 GHz) and the energy-efficient 65 nm Athlon X2 BE-2400 (2.3 GHz). Both are based on the same architecture, but the latter is AMD's latest AMD64 breed, showing that modifications on a manufacturing level can impact the power requirements of the entire system.
If you want to go for an AMD processor, the Athlon X2 or Athlon 64 X2 still are the most reasonable choices. These processors are affordable - some are even downright cheap - and they still deliver sufficient performance. Enthusiasts with a demand for high performance under threaded workloads should look instead for a Phenom (once the B3 stepping is available) or Intel's Core 2 processor family, which still is the champion when it comes to performance, and often the leader in performance per watt as well.