Thank you for that Daniel.
We just finished upgrading the 2 servers with the memory as given above a few minutes ago. I tested the various possible memory configurations, and can confirm that your answer above works perfectly.
I couldn't find where to 'accept' your answer and to mark the question as answered though. Could you possibly do that? Keep the forum nice and tidy :-)
In the end, because we had 2 servers, both with 4 x 4g x 2 processor banks (8 x 4g in total) per server, I configured one server with 4 x 8g per processor bank (64g total) and set it to Advanced ECC mode (8g's were in slots 2,5,3,6 with 1,4 empty as per manual).
For the 2nd server we used the spare memory modules and put 4g modules into every slot, giving 48G. This one was configured in Optimizer mode, again as per manual.
FYI, I also tested with:
A1/B1: 8G
A4/B4: 4G
A2/B2: 8G
A5/B5: 4G
This you told us would work and we confirmed it - and then also added:
A3/B3: 4G
A6/B6: 4G
You said it would work on 8G/8G, not with 4G/4G but we tried it anyway. This actually did work, BUT was flagged as an "Unsupported Memory Configuration" and gave the "F1 to continue" message. I assume that the memory in this configuration would have been in a very low performance mode or something like that. I suppose that in a pinch you might be able to use it like this (home server anyone?), probably by disabling the "F1 to continue" option in the BIOS.
Thank you very much for your assistance.
Regards,
Ian.