Mac Pro 2008 3,1 memory upgrade As you can discover, there are two riser cards, each with 4 slots accepting ECC FB-DIMM modules with heat-spreaders operating at 800 MHz, and I've just upgraded from 4x1GB, all on riser A to 4x1GB on riser A plus 4x4GB on riser B for a total of 20 GB. My question is whether it is better to mix the 4GB and the 1GB modules (pairwise) on a single riser card or to do it the way I've done it, segregated.

I've had a look around on this specific question, beginning with Apple's installation manuals online, and have no guidance so far, so I suspect there isn't any reason to do things differently. In fact, the new 16 GB upgrade was not recognized right away and I had to swap the two riser cards, top for bottom, to see all the RAM. I've done one round of testing using Rember (the GUI for memtest) and it has run tests through "Bit Spread" without registering any errors in the 17 GB or so that it is testing.

My inclination is to leave well enough alone, but I am curious as to whether my installation the best for the long run both with respect to heat dissipation and memory access.
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.