Mac Pro 2008 3,1 memory upgrade
The 'pairs' on a 3,1 are rather confusing &amp; don't map how you would expect.

The common confusion is to think that 'a pair' is one stick in the same place on each riser - because that's how the machine shipped when new.

From Apple KB - Mac Pro (Mid 2012 and earlier): How to remove or install memory

If you have            Install

Two DIMMs              One on the top memory card and one on the bottom card
Four DIMMs             One pair on the top memory card and one pair on the bottom card
Six DIMMs              Two pairs on the top memory card and one pair on the bottom card
Eight DIMMs            Two pairs on the top memory card and two pairs on the bottom card


So, in essence you currently have 2 pairs of 1GB &amp; 2 pairs of 4GB &amp; you have them mapped to the best configuration for dual-channel memory.

There should be no difference if you mix the pairs to have 2x1 &amp; 2x4 on each riser, so long as you keep each as a pair, next to each other, on any riser.