Mac Pro Memory: A Hot topic

The MacPro uses a new type of RAM modules, so called FB-DIMM (Fully Buffered Dual In-Line Memory Module). In summary, RAM modules are no longer working in "parallel" but in "serial".
It has many advantages, the first one being to simplify motherboard RAM circuitry: only 2 RAM slots to accommodate 2 riser cards on which 4 RAM modules can be installed for each. So the MacPro has 4 memory channels, but Apple could have used up to 32 instead of 8 slots (with the limited interest).
The main question is: How to properly install RAM modules to benefit from this new technology? We think that the analysis started with Anandtech's article, and completed by Barefeats.com and diglloyd.com are worth reading.
In summary, Apple does not provide the optimal setting for its MacPro regarding the RMA modules installation: instead of 2x512MB on a single riser card, one should have 2x256MB on each riser cards for a total of 1GB RAM in both cases. In other words, full memory speed can not be achieved until 4 modules are installed correctly over the 2 riser cards; so called "quad channel" setting.
Regarding performance gain of such optimal setting, it is very variable from 0% to 25% depending on benchmark settings (see Barefeats article.
So if you are using a RAM-dependent application, then you will want to ensure that you have the optimal quad channel set up; if you mostly use CPU-dependent application, then it won't matter that much.
JCJ