8GB is a good place to start with since on most motherboards, that leaves you with two more slots for later if you find out you need more. If you have a tendency to juggle multiple games or applications, 16GB may also be helpful since it increases the amount of spare RAM either for disk caching and reducing swapfile use.
In my case, I'm using over 16GB much of the time and RAM was only $100/16GB when I built my current PC so I simply went almost straight for 32GB... that's the first time I spent less than $400 on RAM for a PC so I really did not care about splitting hairs to save a few bucks there - I would have ended up maxing up RAM sooner or later anyway, might as well do it while RAM was still near an all-time low and get the most...