When it comes to memory overclocking, particularly on AMD platforms, it's all about finding the perfectbbalance of speed (Frequency) and latency (timings). DDR3-2133 is a great overall frequency, but if you have to run the memory at 13-15-15-35 to get it, you are worse off than you would be at 1600 9-9-9-24.
Generally speaking, each 266mhz (133mhz actual) jump in frequency should not cost you more than one bump up in each of the four primary timings. Any higher and the frequency gained is offset by the latency lost. This is because the actual latency of your memory is a result of both the frequency and the timings.
I typically start by putting everything to the rated specs of the kit, either manually or via XMP.
Then, start by increasing the memory multiplier or FSB (multiplier results in big jumps, FSB allows extremely fine tuning). For example, I would go from 1866 10-11-10-28 to 2133 (or 2000) with the same timings. If it's stable, you can try to increase the multiplier again (unlikely to work), loosen each timing by 1 and increasing the multiplier (50/50), increase slowly via the FSB until you find the maximum for those particular timings, or you can keep that speed and try to tighten the timings as much as possible.
Personally, I think that 2133 is the best balanced speed for the majority of users, and I would try to get to that speed and get it as tight as possible. Depending on the IC's used in your kit, that could be anywhere from 8-9-9-24 1T to 11-12-12-32 2T, but most likely it's around 9-11-10/10-11-10.
The voltage of the memory itself can be increased as necessary, although I would try to keep it as low as possible just for peace of mind, I have successfully run kits as high as 1.8v or so for 24/7 use.
The memory voltage is preferable to having to increase the IMC voltage, whenever possible, although to get decent speed and timings you will find that increasing your IMC voltage is pretty much a necessity. However, if there's one voltage you want to keep as low as possible during this, it's your IMC as it's located in the CPU and directly influences CPU heat output.
As for the timings, CAS Latency generally has the most significant effect (1st timing), but they are all intermingled to varying degrees and thus keeping them as close as possible is ideal. When dropping timings, start with the first, then the third, then the second, then the fourth. If one of them refuses to drop, move on to the next.
Command Rate, shown as 1/2T or 1/2N, can be thought of as almost a "timings buffer" in that it essentially adds either one or two clock cycles to every, well, command. The difference between 1 and 2 is not exactly earth shattering, but it is worth the effort to get 1T. This may not be possible, however, if you have all four DIMMs populated. Basically, 1T results in better performance while 2T results in better stability. If you are forced to switch from 1 to 2 in order to get from 1866 to 2133, it's a worthwhile tradeoff so long as none of the other timings require more than a single bump.
Then there are the secondary, and tertiary, timing sets. While not as "popular" as the primaries, the effect they have on performance can be just as significant. It's very much worth the effort to get them all setup as tightly as possible once you have hit your desired frequency.
That said, there are a few dozen total, and it is important that you find a good guide (there are plenty on OCN) that not only helps you to learn how to best configure them, but which also teaches you what each one does and how it affects the function of your memory as a whole.
To sum up, I personally love memory overclocking and find it extremely rewarding, but while it certainly has its performance benefits, they're not typically as immediately apparent as are the results of overclocking a CPU or GPU. I would actually recommend that you leave the memory for last, overclocking the CPU and GPU first, and then tackling the RAM.
Be prepared for a lot of trial and error, but also keep an open mind and be willing to learn. Personally, I struggled with memory overclocking for a long time until I finally took the time to read in depth the wayeach aaspect of the memory affects the others, and what function each timing performs and when it performs relative to the others. Now, being able to mentally visualize what's happening inside the IC's each time I change something, I am able to achieve better overclocks much more quickly than when I was just following a guide as a "to do" list. Different people learn best in different ways, and what worked for me may not work for you, it's just a suggestion.
Good luck, and if you have any other questions, I will do my best to answer!