Ram speed will have very little impact in overall computer speed or FPS in games. Take a look at this article. This is just for example as they are using onboard graphics which gets a much bigger boost from fast ram.http://www.tomshardware.com/reviews/memory-scaling-gaming-haswell-richland,3593-15.html
In your case I would go with 16gb of 1600mhz, find something with tight timings like 9 or less. Use the money you saved on the ram to get a SSD or something.