It is not particularly surprising to see the Athlon 64 FX-57 outperform its single core competitors. The 2.8 GHz part is even powerful enough to compete with fast Intel Pentium 4 processors in their best areas, such as video or audio encoding or rendering tasks. One nice advantage here is SSE3, because it enables performance gains with an increasing number of optimized applications, at no extra cost.
The clock speed increase compared to the FX-55 or 4000+ makes a noticeable difference in many applications, but also entails increased power consumption and a TDP of 104 Watts. Yet in practice, the AMD chips are usually far away from coming close to these numbers.
AMD should be pretty happy, because it is now offering both the fastest single core and the fastest dual core desktop processor. However, with more and more applications being optimized for dual and multi core environments, the need for blazingly fast single core processors will certainly decrease in the long run. This means we can effectively split users into two categories.
Group one is very performance-aware and runs applications that really stress the whole system - gaming, audio/video processing and professional graphics are excellent examples. If you belong to this group, the Athlon 64 FX-57 is a good overall choice, although you can get better performance in some of these categories from other CPUs.
Group two consists of everybody else, people who have occasional workloads that take a lot of time, but who consider delays in everyday work annoying. These people simply require the system to be responsive and ready for anything new they might want to do. In this case, a dual core machine is certainly the better choice. However, the 'entry-level' Athlon 64 X2 4200+ is more than double the price than Intel's Pentium D 820.
We think it is great to see AMD becoming a driving force in the high-performance and enthusiast desktop spaces, but how many people can actually afford to buy their state-of-the-art products? A reasonable dual core processor is something the portfolio is currently lacking. And after all, it was ideal price/performance ratios that initially made AMD successful.