Scroll down to figure 3-2 on this page.  It shows how to put 32GB in this system. There are some major differences (you can't just add the 2 DIMMS and leave the rest alone as you're having to revert from triple channel to just dual channel (put 2 DIMMs in each of the 2 channels instead of 1 in each of the 3 channels).

Hi - thanks for the reply.

That is the diagram I have been using.  The problem is that I have to either put both new strips in one channel, or one in each (A & B)

When I use the following configuration:

CD0 CD0 000 CPU1 CPU2 000 0DD 0DD

I get the BIOS error "Unsupported configuration, DIMM configuration on each CPU should match" - and have to press F1/F2, the systems starts but is unstable (shuts down irregularly).

If I use the following configuration:

CD0 DD0 000 CPU1 CPU2 000 0DD 0DC

I get the BIOS error "Unsupported configuration, DIMM mismatch, the following should match in geometry A1 & A2, B1 & B2"

So I am stumped, could it be dodgy memory?

Try this configuration

DC0 D00 D00 CPU1 CPU2 00D 00D 0CD

The Nehalem processors prefer to have all 3 memory channels filled per proc