Hello igmac
With 8x4GB and 4x8GB modules available the maximum configuration would be this:
A1/B1: 8GB
A2/B2: 8GB
A4/B4: 4GB
A5/B5: 4GB
Available memory will be 48GB unless my math isn't working this morning. If you had 2 more 8GB DIMMs then you could populate the third slot on each side; excluding limitation on quad rank and other restrictions on using the third slot.
Thanks