To answer your question about RAM speed affecting performance. Most setups don't notice much of a difference between 1066, 1333 or 1600MHz memory. Benchmarks will show differences, but in real life the difference is much less noticeable.
If you're adding memory and aren't using all available memory channels (3 per CPU for 11th gen servers, and 4 for most 12th gen servers), I would recommend to add memory in such a fashion to use all memory channels.
It is also important to properly populate your memory in your server.
To help figure out how to populate the memory, check out this site: http://poweredgecpumemory.com/. It lets you pick your server, the memory quantity you want and show you some of the dimm population options.