Bare Feats published a couple of great articles that take a close look at Mac Pro memory performance.
The first article compares the performance of two, four, and eight matched FB-DIMMs in the Harpertown Mac Pro (i.e., the latest Mac Pros). Rob-ART found there's a significant performance advantage to having all eight memory slots populated on the latest Mac Pro.
The second article compares the performance of 667MHz FB-DIMMs in both the Clovertown and Harpertown Mac Pros against the performance of 800MHz FB-DIMMs in the Harpertown Mac Pros. While it's obvious that 800MHz is faster than 667MHz, what's surprising is the significant difference between 667MHz in the Clovertown and 667MHz in the Harpertown. This performance difference really highlights the chipset improvements in the new Mac Pro.