I have a Mac Pro running dual Xeon X5675s (3.06 Ghz). I purchased 8 16GB PC3L-8500R DIMMs in the hope of upgrading it to 128GB. It sees all the memory, but it's only running it at 800 Mhz (PC3-8500 should run at 1066 Mhz). Is this a limitation of the amount of memory, or is it something about these DIMMMs that it doesn't like? I tried another CPU card (also with X5675s) and got the same result. With the original 4GB DIMMs, it runs at the expected speed.
ETA: I'm answering my own question. This is apparently a weakness of the architecture. If I remove the fourth DIMM from each bank, the memory runs at the correct speed.