1600MHz is adequate for most people.
2133MHz can have a benefit in some scenarios.
TIMINGS are sometimes more important than FREQUENCY though it's complicated.
The AMOUNT of RAM is usually more important. If you need 32GB of DDR3 it's best to get inexpensive 1600MHz of good quality.
2133MHz is overkill for most (up to 15% in the best scenarios versus 1600MHz). 8GB is optimal. 16GB is pointless for just gaming.
If this is for gaming, get 8GB of 2133MHz from G.Skill.
Don't get hung up on the small things like what does the "N" stand for. If you care, go to Wikipedia.
*Next time you have a request like this, state your:
- Purpose (gaming/video editing?)
- Basic Specs (Win 7 64-bit, Graphics Card, CPU, Motherboard)