When I shop for RAM, quantity is no less important than its speed as well as compatibility with the respective motherboard. Be sure to match up your RAM with the QVL; it's partly marketing but does indeed come into play with increasing numbers of DIMMs.
As to 8 vs 16gb, The OS will generally reserve half your RAM, but, even so, gaming doesn't require much in comparison. What's more often overlooked is a video card's VRAM, but 3gb will do you just fine for the time being; in fact, by the time anything out there truly challenges that, you'd want to upgrade the video card regardless.