Yeah, i kinda overstreched it a little with the number/performance indication. Sorry.

But i would say that for applications coded to work with threads and multiple cpus, the dual cores could perform double. The bigger overhead is lessened by faster and larger cache memory and a faster fsb (and hyper transport in AMD's case).
Sure it's not 100% double the speed, but it's close enough.

All work and no play makes me..., well, you know the rest.