Don't know if TC is still around but I made similar topic a bit later on and got a great answer:
http://www.tomshardware.com/answers/id-3535046/meaningful-gains-gaming-performance-16gb-3200mhz.html
I will just buy one brand new kit of desired size and speed to replace my existing one. Easier than taking the risk of buying a different small one that ends up not working with my current one, then I have 2 small ones that I don't really want which would be really bad if they were 4GB. Coming from an 8GB user you definitely want to look into getting more than 8GB of RAM. I suggest buying
16GB(8x2). I also strongly suggest moving to
DDR4 as I noticed DDR3 prices have gone way up presumably because they're EoL. It will probably be much cheaper to just buy DDR4.
You might need a new motherboard if yours is old. I'm not sure since all the ones I looked at support DDR4(skylake & kaby lake motherboards).