In March 2012, I upgraded my MacPro from 16GB @ 1333Mhz (4x4GB OWC ECC RAM) to 32GB @ 1066Mhz(4x8GB G.Skill non-ECC ram). Whilst the boost over 16GB helped my workflow from hitting the VM, I was never happy with the drop in RAM speed. Resetting the SMC/PRAM has no effect on getting the speed back to 1333Mhz. While the ram could easily be clocked to 1333Mhz in a PC using manual controls, it was a no-go on the 5,1. Today, with a loss of 8GB, I was finally able to run 24GB @ 1333Mhz by dropping a 240-Pin DIMM from Slot #4(closest to the heat sync). Apparently slots 1 & 4 share bandwidth and adding chips from most vendors slows the bandwidth to 1066Mhz. From my findings, OWC/MacSales chips run fine in 4x 1333Mhz specs. So if you are running 32GB @ 1066Mhz and want a boost to 1333Mhz and can handle loosing 8GB without a slowdown, this is your chance to gain back a few percentage points in lost performance.