As we mentioned and proved in our previous articles, the number and configuration of FB-DIMMs in your Mac Pro can affect performance. The Intel 5000X chipset in the Mac Pro features two 144-bit FBD branches, each being the width of two FB-DIMMs (effectively giving the chipset four "channels"). Therefore you need at least two FB-DIMMs in the system (the width of a single FBD branch), but ideally you'd need at least four to have a hope of attaining peak bandwidth.
So... make sure If you get a Mac Pro that you have at least 4 memory modules in it...
That's because the system is fully 64bit... with a 256 bit wide bus. Each of the cores can have full access to a segment of memory.