PC6400 RAM

Unless the memory controller and ABM can be taught to run above 667MHz and at the proper voltage... No and highly unlikely. And does it use ECC FB-DIMM CL5 with proper spreaders and thermal characteristics?

I would use one of the memory selectors specific for Mac Pro rather than generic to be safe. It is hard enough for Crucial even to insure that every DIMM shipped will meet and work with other Crucial and other brands.

No. The Mac Pro requires specifically FB-DIMMs which have different circuitry than standard DDR2 RAM. Very simply speaking… normally the memory controller handles sceduling to your DIMMs. In the case of FB-DIMMs the first DIMM in each channel (the MP has 4) handles the scheduling for itself and the DIMM behind it in the channel.

Be aware that's it's not necessary to buy Apple RAM. There are plenty of other places like Crucial, Kingston, MaxUpgrades, TransIntl to buy your Mac Pro RAM. Many of these will offer better pricing than Apple. However, if you want a single stop for your warranty and serice needs there's nothing wrong with going Apple.