If you need the dimms:
This site will know what to dimms to use with it. Since they sell them refurbished. So you could enquire of them if you should buy a kit of 4x 1333mhz dimms for 16gb since we don't know what dimms you have.
Or you could identify your dimm part numbers using
cpu-z (spd tab) and search for kits. Well technically you should buy a kit of the full amount of memory you want eg. 4x4gb to ensure the actual dimms are compatible with each other but, ddr3 isn't 'so fussy' as to what memory it works with.
If you could find modules of the same make and model that you already have then they would probably have the best chance of working.
So technically I could only say that a 4x4gb kit would work for 16gb since the memory vendors only validate complete kits and if you only need 2 second hand dimms well you can only say they will probably work but there's a chance they might not.
Or you could get a 4x4gb 1333mhz kit and aim for 24gb and see if they work with your modules.
So how much memory do you have with it? You could have any assortment of 1gb, 2gb or 4gb dimms. If your dimms are not already 1333mhz, then you'd have to throw them.