R5500 memory configuration?

Well, I can find zero on how to configure memory for the Precision R5500.  But lots for all kinds of other Dell systems.  Found one for the T7500 which I think is similar.

http://sivirt.utsa.edu/Documents/Manuals/precision-t7500_Service%20Manual_en-us.pdf

The memory configuration there shows specific types of configurations.  I have two Precision machines with different quantities of memory and one gets better scores than the other.  Both have all the banks full, just one has 2 and 4GB modules and the other 2 and 8GB.  The 2 and 4GB gets better CPU and memory scores.  Both say they are using 6 channels but curious if something is wrong with the 2 and 8GB modules unit that the server doesn't like?

Looking at the above link, it doesn't show any combination of sizes.  all are 1GB or all are 2GB or all are 4GB or all are 8GB.  Or 16.  Is this how it has to work?  Can I not combine sizes in the case even if I have all three in the same channel the same size?

Also is there anywhere that talks specifically about the R5500 and memory config?

Thanks.

JR