JJJ is correct. Better to have the option to upgrade later however I should also point out that there is precious little that most computer users do that would benefit from more than 16GB in RAM except perhaps if you plan to do Virtual Reality at some point in the future.
Depends If money is a big issue this month adding two more sticks of matching 4 gig sticks would save you money. But not all motherboards play nice with all four slots filled with some ram sticks. Currently you are lucky that ram prices are fairly low now so buying high speed low latency 8 gig chips will cost less then a nice mechanical keyboard. Do your homework and look for reviews with the ram you want and the same motherboard you have. Two 8 gig chips are the way to go.
