There is online controversy about upgrading memory beyond what is supported by Dell.  Some have been able to install 8 GB DIMMS for 32GB of memory - I haven't, but was able to install 4 GB DIMMS for 16 GB of memory, after a failed first attempt.  There seems to be something happening in the BIOS (or CPU, according to some) that has to do with what memory was previously recognized before new memory can be recognized.