Since the above posts are somewhat confusing, I'll try to clarify. 90% or more of the performance of the CPU is determined by the final core clock. That can be increased by either increasing the base clock (often FSB on Intel systems) or by increasing the multiplier, or both. Increasing the base clock has the advantage that I/O speeds (including RAM or FSB) are also boosted, and the disadvantage that OCing more of the system increases the chance of encountering a problem.
Since historically core clocks have significantly outpaced I/O, current CPUs are designed with big caches to minimize the effects of relatively slow I/O. Thus boosting the RAM or FSB speed doesn't usually help a lot, but it might give you an extra 10%, everything else being equal (which it often isn't!).