[NAME] First, you should know that 48GB is not a Dell tested configuration according to the Setup and Specifications. Second, memory should be installed in match pairs in the slots with the same colored securing tabs. Third, the maximum speed the memory will run at in the XPS 8920 is 2400 MHz, so buying faster memory is a waste of money. If you are looking for guaranteed compatible memory take a look a crucial.com. And finally, will it work? I searched this forum for an example of 48GB in an XPS 8920 and found none, but someone got 48GB to work in the XPS 8930. There are also examples of folks getting configurations not listed in the Dell memory configuration matrix to work, probably the most common example is 32GB (2 x 16GB). Good luck.
I have an XPS 8920 that currently has 2x8GB of ram. I would like to add 2x16GB of ram for a total of 48GB of ram. It would fill 4 out of my 4 available DIMM slots. The question is: can mixing memory sizes (not brands) work for my computer? Please get back to the discussion asap. Thanks. 
XPS 8920, 2x16GB + 2x8GB = 48GB?