What RAM should I buy?

There is memory speed, and then there are memory timings.

Speed is the frequency that the memory operates at (in MHz... and remember that when you are looking in CPU-z that ram is DDR [Double Data Rate], so the frequency that it says will always be half of what it is actually running at). It is fairly straightforward and easy to understand.
And remember that the ram speed is tied to the reference clock, and in this way you can easily OC your ram beyond the stock speed.
Speed is mostly important for higher bandwidth, as it just means that the ram can move more information.

Timings are different, and should not be confused with the speed. Timings do not effect the speed (in MHz) of the ram at all.
Instead, they change how “efficient” the ram is. In other words, the timings affect the “turnaround” of the information.
The timings are generally listed something like this: 5-5-5-18. While I wont go into all of the details about what those each mean here,
I can give you a basic idea of what the timings mean. Timings change how long the ram will wait to do something.
So having ram with looser (numerically higher) timings, means that the ram will “wait” longer in between processes.
Using the example of 5-5-5-18 ram, information is copied, it then waits for 5 clock cycles before it moves onto the next step.
Ram with 7-7-7-23 timings running at the same speed (in MHz) will move the information there just as quickly,
but then will need to wait for 7 cycles between doing things, rather than 5.
This is why “performance” ram is in fact faster.