Just remember that the CPU is ONE component. You also need to consider how much memory you have, how fast your hard drive is, what your network speed is, etc. The system will only run as fast as it`s slowest component.
Often you will get better bang for your buck by optimizing your software rather than throwing hardware at it. Rasmus Lerdorf (the PHP creator) did a presentation on taking an application from 17 requests/second to 1700 requests/second by optimizing the sofwtare.
Blog - http://www.buggy.id.au/