An interesting set of simulations at Sandia of multi-core systems have been reported:
A team of researchers simulated key algorithms for deriving knowledge from large data sets. The simulations show a significant increase in speed going from two to four multicores, but an insignificant increase from four to eight multicores. Exceeding eight multicores causes a decrease in speed. Sixteen multicores perform barely as well as two, and after that, a steep decline is registered as more cores are added.
The reason for this is fairly well known, but it’s nice to see numbers put to the effect..
The problem is the lack of memory bandwidth as well as contention between processors over the memory bus available to each processor.
The original Sandia press release has more information.