16GB DIMMs in 2010 Mac Pro 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.
It can run 64GB as dual channel [8's] or 96GB as triple [16s].

From EveryMac - Silver-Colored Mac Pro Q&amp;A


  Actual Maximum RAM
  
  Although "Mid-2010" and "Mid-2012" Mac Pro systems with a single
  processor, which EveryMac.com notes as the "Quad Core" and "Six Core"
  models, officially support a maximum of 16 GB of RAM, as discovered by
  site sponsor Other World Computing, they actually can support more.
  OWC first noted an actual maximum of 32 GB of RAM and later increased
  this actual maximum to 48 GB of RAM.
  
  Systems with dual processors, which EveryMac.com notes as the "Eight
  Core" and "Twelve Core" models, likewise officially support 32 GB of
  RAM, but again, OWC found they actually can support more. OWC first
  confirmed an actual maximum of 64 GB of RAM running Mac OS X 10.5
  "Leopard" and later increased this maximum to 96 GB of RAM running Mac
  OS X 10.6 "Snow Leopard" or higher. More recently, OWC yet again
  discovered these dual processor models can support up to 128 GB, but
  only when running a 64-bit version of Windows XP or later or Linux.
  Mac OS X only can support 96 GB of RAM.