Make sure you have the latest version of BIOS (A10) running.
Early versions of BIOS for the XPS 8500 mis-identified 1600 MHz RAM as 1333 MHz RAM. This was fixed in BIOS version A06 and later. You can update directly from whatever version is installed to A10, without updating to any of the in-between versions.
Check out crucial.com too. They guarantee their RAM will work in the system for which they recommended it, or they take it back.