Barefeats keeps comparing the new Nehalem-based Mac Pro with previous models. So far, they concluded that 8 Core Mac Pro are faster than single Xeon-based configuration, till they run a Photoshop test. Indeed, in this last test, the 4 core single CPU Nehalem Mac Pro is always about 4% faster than the high-end Nehalem hardware. It is not only true for Photoshop, but also with iTunes. They might be several reason, and one can speculate that the high-end model spends too much time in distributing the task to the different cores. In addition, as nicely demonstrated by our friend Lloyd in his recent review of the Nehalem Mac Pro, CS4 rarely uses more than 3-4 CPU cores effectively and usually only for a short while.
In other words, it might be worse testing the single CPU model as well as the high-end with your common applications before investing into the top-notch model which might not provide the expected gain of performance, as most of your applications might not be coded to benefit from more than 2-4 cores.
In addition, unlike reported by Apple, the new single CPU Mac Pro can manage 16 GB or RAM composed of 4 GB modules.
