I don't think there's enough difference between 1333/1600 and 2133/2400 to warrant all the headaches. However I'm sure there are plenty of people that will disagree with that statement.
With regards to future proofing, there really is no such thing in the PC industry.
But specifically with these speeds, if there was something in the future that magically brought the latencies down, it's probably going to be DDR4 or something like that. E.g., not compatible with your motherboard. Therefore, pick the speed you want today, and don't think about the future too much.
Regarding 8GB vs 16GB, 8 is plenty enough for any games. And many will say that more than 8GB actually slows games down ever so slightly.
But if you're worried about the future, buy 2x 4GB and a motherboard with 4 sockets. Then you have 2 slots extra to buy more in the future, should you feel that you need it. However, I doubt that you will, unless you start running virtual machines, or SQL databases and alike.
Hope that helps a little.