This could well be my ignorance, but I recently upgraded my Early 2009 Mac Pro 4,1 2.66 Quad Core to 5,1 3.46 6 Core. Upgrade went perfectly. Had only 12GB of 1066 Memory in 3 slots (3x4), replaced with 32GB of 1333 in 4 slots (4x8) (OWC said this would be fine). Seems to run fine, but bus speed still shows as 1066. Did some Googling and couldn't quite get clarification on why this was happening.