When we compared AMD's Athlon 64 X2 and the new Phenom 9000 quad core processor last month, we forced Windows to utilize only a single processing core, in an effort to understand the real performance differences between the AMD64 architecture and the Stars architecture of the Agena/Barcelona core. Depending on the particular benchmark, the results made clear that Phenom is indeed up to 20% faster than Athlon in a core to core comparison, thanks to its optimizations and the L3 cache. In addition to these performance gains, Phenom's unified quad core design and smart cache architecture should provide more benefits under heavily threaded conditions.

http://www.tomshardware.com/2008/01/...ling_compared/