Mr. Bennet-Alder:
I think you are confusing DDR3 and DDR2 type of memory modules.
DDR2 PC2-8500 were the FBDIMMs that were used in the 2.1 and 3.1 Mac Pro versions (before 2009).
These were 800Mhz or 600Mhz versions. They used serial data channels from one DIMM to the next so the DIMMs had to be clocked very quickly to achieve speeds. Thus, the need for the fully buffered DIMM to pass the data from one DIMM to the next. Because they were clocked so quickly, they had to have heat sinks to bring the temperature down to the lower fan speeds Apple wanted to use. The heat sinks kept prices high. So did the fact that few servers adopted the FBDIMM technology. These also had ECC.
DDR3 PC3-10600 and DDR3 PC3-8500 were used in later MAC Pros, version 5.1 and 4.1 respectively.
PC3-10600 is a 1333Mhz DIMM. PC3-8500 is a 1066Mhz DIMM.
Both of these are using ECC bits. Thus, there are actually 9 parallel data bits, one of which is for ECC.
Registered/buffered and Unregistered/unbuffered have a different meaning than FBDIMM.
It is better to use the Registered/Unregistered terminology to avoid confusion with FBDIMMs.
Registered DIMMs are sometimes referred to as RDIMMs and Unregistered DIMMs as UDIMMs.
Also one may see an R on the end of the PC3-8500 designation as in PC3-8500R to designate registered memory.
One thing that is for sure, the memory controller in the CPUs has to have uniform memory in the slots for each given processor. That is, you cannot mix registered memory in one slot with unregistered memory in another slot for the same processor. Apple indicates the computer will not boot in this case. This is because the signal timing differs to each and the memory controller in the CPU is only a homogeneous type.
Apple indicates that registered DIMMs are supported by Mac Pro version 5.1 - 2010 models.
They have not provided the same indication for Mac Pro version 4.1 - 2009 models.
WIth that said, firmware on version 4.1 macs may be able to be upgraded to support RDIMMs.
I suppose this updates the timing issue to suppor the RDIMMs.
Why is this significant -
Used registered DIMMs with ECC are easier to come by due to people upgrading their server computer to more denser memory capacity in the same slot.
Used unregistered DIMMs with ECC are harder to come by.
Accordingly, used registered DIMMs are less expensive on ebay.
My 2 cents.
