Apple Mac Pro (Early 2009) - Manual (Page 47)

Chapter 3 Making a Good Thing Better 47
If you install equal-size DIMMs (all 1 GB or all 2 GB) in your Mac Pro, Apple recommends
that you fill the slots in the order listed in the table.
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.
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)
Slot 4
Slot 3
Slot 2
Slot 1