The FX-55 is AMD's top performer. It's a 2.6GHz beast that will eat through current and future games and applications. Its architecture is based on that of the FX-53, which in turn was based on the first generation FX-51, and AMD has come a long way in bringing 64-bit to the desktop. The major differences between the product refreshes was the move from a 940 to a 939 platform, and dropping the requirement of registered ECC memory which was expensive and designed for servers. The FX-55 however, will operate happily on any consumer 939 socket motherboard with DDR memory up to PC3200.
Moving onto performance, the FX-55 offers some advantages over Intel's Prescott series, but falls behind in other areas like PCMark. As the FX range lacks the SSE3 optimisations, it lagged behind Intel's 3.73GHz behemoth by a huge 1000 points -- enough to provide that little extra performance in gaming, video-encoding or any other CPU stressed activity.
Our 3D tests told a different story. The FX-55 kept pace with the Extreme Edition in 3DMark01's software transform and lighting, but fell behind the 4000+ in 3DMark03, losing its edge by 77 points. It recovered to take out the fastest run time and highest average frame rate in Bunker, only to be pipped by Intel in Treehouse.
Ultimately, this is a great gaming focused CPU and you'll pay for the privilege of owning one. If $1669 is too much of an investment, then check out the 3200+ and 4000+ models.