pro-gamer :
16gb can boost up your pc well if you want to do some extra stuff like photoshop,editing etc but it gives you about 10%performance in gamming.
If all the available ram is superior than the game max memory usage coded in it, it's simple IMPOSSIBLE to get any performance gains.
So this 10% gain is completely irreal, and random.
Few games uses more than 4gb. So I would say 8gb is more than enough if you only gaming.
I, however, only do gaming and have 16gb.
So you guys would ask: Vitor, why do you have 16gb memory?
Answer: Cause just wanted to piss off some friends saying I have more ram, and I had money at the time to burn in hardware.