With RAM speeds, except in specific hardware configs and usage cases, speeds don't matter. Get the bog standard 1600Mhz CL9 1.5v RAM and you will be fine.
Capacity depends on what your doing.
- General usage (Family PC type stuff) you can get away with 4GB.
- Gaming, then get 8GB.
- Video editing, number crunching, rendering or pretty much any professional application, 16GB.