Help with memory upgrade - PC from 2013

You've got 2133 so guessing a K model CPU, best bet actually would be to get a set of 2x8GB 2133 or 2400 (or higher if OCing the CPU). Trying to 'add' DRAM is a crapshoot - even if you get another set of the same exact model - you're going to end up getting DRAM made with a different lot of PCB, solder and probably even different memory chips being used (they've changed them on the RJ X a few time - this series was released back around the end of 2010 aimed at the release of Sandy Bridge in Jan 2011. So 2x8GB in 2133/2400 or higher depending on CPU, the RJ X s are good on Haswell, as are their Sniper line. Best I've found on Haswell are their Trident X line of sticks