Apple Mac Pro (Early 2009) - Manual (Page 56)

56 Chapter 3 Making a Good Thing Better
If you install equal-size DIMMs (all 1, 2, or 4 GB) in your Mac Pro, Apple recommends
that you fill the slots in the order listed in the table.
If you have Fill these slots
Two DIMMs 1 and 2
Three DIMMs 1, 2, and 3
Four DIMMs 1, 2, and 5, 6
Six DIMMs 1, 2, 3, and 5, 6, 7
Eight DIMMs 1, 2, 3, 4, and 5, 6, 7, 8 (all)
Slot 4
Slot 3
Slot 2
Slot 1
Slot 5
Slot 6
Slot 7
Slot 8