T5810 Memory configuration

I have a T5810 with 8GB of memory. (two x 4GB modules)

They came from Dell in slots 1 & 2

I want to add two x 16GB modules.

The manual says: Memory modules of different sizes can be mixed (for example, 2 GB and 4 GB). But, all populated channels must have identical configurations.

But does not say which of the eight slots are for what channels. Four slots have white ears, four have black.

It seems to me that the system does not care what modules are in which slots, so long as they are installed sequentially. I have tried all sorts and the result in the BIOS is the same- 40GB of memory, 4 active channels.

I think Dell have just copy and pasted some generic rubbish into their manual. The information is just inaccurate and unhelpful.

I contacted a service agent and they didnt have a clue either.