The R510 does not support using the 8 and 16gb dimms when you have the cabled drives, it is due to power output constraints. With hot swap drives and a backplane it doesn't have the constraints and will allow you to configure with the 3 16GB dimms.

Let me know if this answers your question.

Thanks Chris, that is a logical explanation.  The tech support person I spoke with seemed to think it might be a matter of BIOS version, since, apperently, I can order a brand new R510 with a cabled drive and 16GB dimms.  

Is there anything that can be done to ugrade this R510?   Lower voltage memory, for example?  We need much more memory than 4GB dimms will permit and we bought it with expansion in mind.

What is the effect if I put 16GB dims in there?  Will it not post or will it work but potentially crash on high power loads?

Thanks.

I went back to the R510 configuration page and I noted that the memory line reads:

   48GB Memory,(3x16GB)1333Mhz,Dual Ranked LVRDIMMs at Std Volt, for 1 Processor Optimized

I assume LVRDIMMs mean "low voltage".  Is this the official solution?

It works.  Having gotten widely conflicting answers from different people at DELL, we just tried it.  It boots and works fine.  

Thus the mystery deepens as to what the true limitation is.   Are only 1.5V 16GB DIMMS a problem?  Will 1.35V DIMMS still work with 96GB and dual processors on the -4 system?  If it is power supply dependant, then what is the actual limit -- total power draw, or DRAM power draw?