How does it all work together?

What is the bottleneck in a standard system besides the RAM? Say I was going to increase processor speed, had a HD that was 5400rpm, motherboard with a 100mh FSB, and 256 Mg of RAM. Would there be a limit on the processor speed that (i.e. a waste of money going faster) given the above specs? HOw do you deside which component would do you the most good, and would you get the maximum value given all the other parameters. How dows it all work together? Any articles out there that address this topic?