TheITSource. While the R815 didn't actively support 16GB dual Rank dimms at the time of it's release, it does support them currently. It looks like support was added for the dual rank 16GB dimms around the end of 2011.  One thing I would verify before trying to install them is that the R815 BIOS and iDrac are up to date and current.   
Hello, I am hoping to find a little more recent info on the supported RAM configurations for an r815. The latest info I could find is this 2010 dated doc here: www.dell.com/.../Poweredge-R815-Technical-Guidebook2010.pdf According to this document, there are no supported RAM configurations employing Dual Rank 16GB DIMMs.  Specifically note the 256 and 512GB configurations which all call for either dual rank 8GB DIMMs, or 16GB quad rank DIMMs. What I would like to know is this: Is that a hardware limitation that persists today, or was that sheet a function of the type and size of DIMMs available at the time of publication?  Going to QR DIMMs means dropping RAM speed to 1066MHz, so there is a penalty to be paid for using QR DIMMs in any config.  Can anyone tell me with certainty whether this limitation still exists?  Thank you for your time!
Supported RAM config of r815 - Dual Rank versus Quad Rank