cr - to clarify a bit:
All data to/from RAM has to go through the memory bus and also pass through the FSB. Thus, whichever has the lower throughput is the bottleneck. Normally, that is the FSB. Thus, we want to match the throughput of the FSB with the throughput of the memory bus. For example, with a 1333MHz throughput FSB, we want 1333MHz memory throughput. Since almost always the memory bus will be running in dual-channel mode (essentially doubling the width/throughput of the memory bus), each individual memory module needs to run at half that throughput, or 1333MHz / 2 = 667MHz throughput in this case.
Thus, just take the FSB throughput and divide by 2 to get the DIMM module speed you will need.