if that is single stick then second one will get you most benefits.
ram will be in dual channel mode and effectively double the bandwidth.
THEN I would get second ssd to have OS and others separated. As my usual problems was Windows update downloading 10GB without any consent.
16GB of RAM is ENOUGH, if the most demanding thing you do is playing games. If you do serious 4K editing and developing stuff, you may need 32Gigs.
Make sure you have a 2x8 instead of 1x16GB RAM. Dual channel is very importand and will give you a 20+ % performance increase in games. With this CPU you can have up ti 3200Mhz RAM, (even more if you overclock it carefully).
Also, as previous comment said, you should buy a second ssd to seoerate wi diws from games. (256 gigs for everything isnt to much too)