Some games will slightly benefit from faster memory, but it's also going to depend what CPU you have. Your System RAM (DDR3) simply stores data which the CPU reads and processes. If the System RAM is too slow, the CPU isn't fully used which reduces performance.
So you generally need a good CPU that's overclocked and one of the games that actually can benefit to see much difference.
In general, there aren't many scenarios that benefit above 1600MHz but they do exist. Considering the fairly SMALL difference in cost, I recommend 1866MHz CAS9.
16GB vs 8GB:
There's little evidence that more than 8GB will be a benefit for GAMING for a long time. Unless you do video editing (not converting) or use Virtual Machines or another task you know benefits, that memory is really wasted.
Good 1866/2133 memory costs roughly $80 to $100 dollars for 8GB. This is also about the cost of a good SSD like the 120GB Samsung 840 EVO and that's something that would greatly benefit you as your main Windows drive instead of a hard drive (have a large hard drive for games and storage as well).
Examples:
1) 16GB (2x8GB) of 1600MHz CAS9, for $135:
http://pcpartpicker.com/part/crucial-memory-bls2kit8g3d1609ds1s00
2) 16GB (2x8GB) of 1866MHz CAS9, for $158:
http://pcpartpicker.com/part/gskill-memory-f31866c9d16gxm
3) 8GB (2x4GB) of 1866MHz CAS9, for $78:
http://pcpartpicker.com/part/gskill-memory-f314900cl9d8gbsr
If you get a 2x8GB kit, make sure your motherboard supports that (32GB and four slots).