It depends on the system you are running.
If you have a Core 2 Quad and 667MHz RAM then I would probably prioritise RAM overclocking or buy faster RAM, as it will be limiting performance in some situations.
However, on the other end of the scale, an X58/ i7 system has more than enough memory bandwidth even at motherboard default speeds, so running the RAM at spec is fine and focusing on CPU speed is the best way to go.