Here's my advice:
1. Before thinking that high-MHz RAM is faster, take the MHz and divide it by the wait states (CAS.) You will see that "faster" RAM is usually not faster.
2. If You want fast RAM, think multichannel, not fast-MHz. Many people do not even read the manuals that come with their motherboards. If they did, they would populate all their DIMM sockets to enable all the channels.
3. For the maximum number of RAM channels, You will need to run high-end Intel processors (Xeon) that support quad channel RAM. Beware: This RAM is really expensive, especially when it's rate to run a bit faster.