Memory is a very fascinating thing. People will spend hundreds and hundreds of dollars more for higher frequency ram, when in reality they run just about every task the same way. There are very few things that will actually be able to take advantage of higher frequency RAM. To explain; what you should consider when buying RAM is the frequency, the Cas Latency (CL), as well as the price.
First, let's cover frequency. This is the speed of the ram measured in MHz, and despite common belief, even if you spend hundreds of dollars more on a higher frequency RAM, without the proper Cas Latency backing it, it could all be a waste of money.
Cas latency is the amount of cycles the RAM has to undergo before it is able to complete the task.
Let's take these two definitions and apply them to buying ram. You see a RAM stick that is running on DDR3 1600, with CL 9. To test how fast that will run through a task, you do 9/1600 and receive 0.0056. This is the amount of time (in seconds) it will take to complete a task. Now, you stumble upon DDR3 2400, CL 11. You would do 11/2400 and receive 0.0045 seconds. If you want to spend another hundred or two for 0.001 seconds of speed, then be my guest. It's just not a smart choice if you are operating on a budget.
I hope this explanation helped. If you have any further questions feel free to message me.
The 1866 is not that much more than 1600, not a bad choice, the 2133 does not buy you the performance it cost so I would pass on it. 1600 or 1866 would be a smart buy, CL9 or CL8 is what you want to have, hope this helps..
You want to look for the highest freq with the lowest CL at the best price - it also depends on what you, high freq won't be of much of an advantage if all you do is game or single task, high freq comes into play when multi-tasking and using memory centric apps like those for imaging, video work, CAD, GIS, VMs, et al or apps using large data sets