Barefeats continues its test of the new Mac Pro comparing it with the previous models. They mostly concentrate on pro applications. Those results indicate that the new Octo Core clocked at 2.26 GHz is faster than the previous high-end Octo Core sporting 3.2 GHz Xeon. The new high-end Mac Pro featuring Nehalem Xeon at 2.93 GHz is defining a new performance reference. In one test, it completes the task in 63 seconds vs. 102 seconds for the previous high-end model. If the new architecture of the CPU has evolved and may contribute to this performance gain, it is mostly linked to the new memory architecture offering better efficiency than the previous FB-DIMM-based system. Barefeats measured that the memory bandwidth is larger if you use 3 modules per processor instead of 4. The Nehalem is optimized with 3 memory channel. The difference is rather huge, as one can reach 9261 MB/s with 3 modules vs. 6195 MB/s if you use 4.
If you are looking for the best performance level you should think about RAM modules when purchasing your new Mac Pro. A model equipped with 6x2 GB RAM modules will be faster than one having 8x2 GB modules, but only if you do not saturate the 12 GB of RAM available, otherwise performance will drop.