I would be buying 1 stick. I actually wanted to add the ram, with the existing ram I have, tallying up 16gb of ram, since I do a lot of multi-tasking. Do you think mixing Corsair XMS3 and G. Skill, the product I linked, would run into any problems, if I did that? The motherboard I have does say it can hold up to 32gb of RAM. I rather not fill all my slots, as I saw the Corsair Vengeance, but want to keep at least one slot free.
It may very well work with some bios tinkering.
Personally i've mixed ram kits before but only from the same maker and matched as closely as possible.