Apple Mac Pro (Mid 2010) - Manual (Page 49)

49 Chapter 3 Making a Good Thing Better
If you install equal-size DIMMs (for example, all 1 GB DIMMS or all 2 GB DIMMS) in your
quad-core or 6-core Mac Pro, Apple recommends that you fill the slots in the order
listed in the table.
Slot 4
Slot 3
Slot 2
Slot 1
If you have Fill these slots
Two DIMMs 1 and 2
Three DIMMs 1, 2, and 3
Four DIMMs 1, 2, 3, and 4 (all)
Note: If you install different-size DIMMs in your Mac Pro, follow the order in the table.
If the DIMM configuration you install doesn’t provide optimized performance, the
Memory Slot Utility appears onscreen and recommends an improved configuration.
To use the utility again, go to /System/Library/CoreServices/Memory Slot Utility.