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.