So I just finished my 4,1–5,1 build. I installed a pair of X5675's along with two sticks of Samsung 16GB DDR3,4RX4 PC3-10600R 1333MHz 240-pin ECC REG Server Ram, for 32Gb. Trouble is, it's showing up as 1066 instead of 1333. Now, the machine is running fine, but I'm curious as to why my memory isn't showing full speed–I triple checked the specs on that RAM before I bought it and thought it should work, and it does, but not at 1333. Tried zapping the NVRAM, to no avail. Anyone have any ideas?
I can't find out what actually decides it, however I just noticed this note on the Apple KB - Mac Pro (Mid 2012 and earlier): How to remove or install memory
Note: Although the memory speed of the UDIMMS is 1333 MHz, they will operate at 1066 MHz or 1333 MHz depending on the processor in the computer.
Typically, MacPro5,1 models with one of the Nehalem processors only support a memory speed of 1066 MHz, while those with one of the Westmere processors support a memory speed of 1333 MHz. In your case, the X5675 processors are one of the Westmere variety, so I can't explain the discrepancy in speed.
I have a pair of X5690 in mine & RAM runs at 1333MHz.
It's probably worth making sure you have the pair in the correct places, just in case...
BTW, the Pros run fastest in triple channel - so if you can afford one more RAM stick...