A situation came up at a client site where a Dell PowerEdge R610 / R710 server was being upgraded to a non-optimal memory configuration. The document Installing and upgrading DDR3 Memory was a short and concise reference for supported configurations.
The following two documents are Dell PowerEdge Technical Guides that also came in handy to when trying to decipher the supported memory configurations.
http://www.dell.com/downloads/global/products/pedge/en/server-poweredge-r610-tech-guidebook.pdf