There is a thread on this already. Yes you can add 2x16 for a total of 40GB, just put then in the empty slots. Make sure the specs match what you need and you should be good to go. The most common are Crucial, Kingston, Timetec and Corsair. In no particular order. I'm using 64GB of Crucial CT2K16G4SFD8266 for almost 6 months with no issues. Before trying Crucial this time, I always used Kingston RAM over the years and never had any issues as well.