The difference in speed is not going to make a big difference in most applications/games. However, with the price or RAM as it is now. I think the difference is only a few dollars, so I would go for 1600MHz if possible. The speed rating really comes in handy when you are overclocking and gives you more room to play.
I don't think any gaming machine needs more than 8GB. You need to have VERY memory heavy applications if need more than 8GB. Video and large photo editing or running virtual machines usually is a reason for 16GB or more.
If you are currently running 1333MHz RAM there is no reason to upgrade to just for the speed.
Yea it is a negligible difference in real world apps. I own Kingston HyperX Blu 1333mhz, and a set of HyperX genesis 1600mhz, I have tried them separately, I cannot tell the difference and there is a .04 difference in cinebench.