OK, so I have a 12-core Mac Pro that has 2 gb RAM sticks in slots 1, 2, 5 and 6. I have two 4 gb sticks to put in. What slots should they go in for maximum performance? 3 and 4 or 3 an 7?
Slots 3 and 7 should give you two 3x2GB triple-channel setups and the remaining 2GB in 4GB modules would run in single-channel. I think that would give you the maximum performance. Third 4GB module would be optimal though since then you could have two triple-channel setups without remainder RAM but you most likely won't notice any difference no matter how you install them (RAM bandwidth is not the bottleneck).