There's so many factors that add up for the final performance... I can think of CPU speed, FSB speed, RAM speed, RAM latency, cache size, L2 cache size, L2 cache speed, special instructions (MMX/SSE/SSE2/etc...), cpu pre-fetching efficiency, number of cpu execution pipelines, number of FPUs within the cpu.... and I'm sure there might be even more that I'm not remembering right now... (And I'm not entering the realm of the I/O devices)

So you see, there has to be a balance between all these factors for the whole to work faster. For example, improving memory speed will only help you until a certain point, if the Front Side Bus can't keep up with the memory output, the memory will just have to work at a slower pace no matter how fast it is in the specs.