Our choice of processor was motivated by the same question. Is there any perceptible difference in performance with a faster processor? We used Intel Pentium 4 processors with the 800 MHz FSB running at 2.4, 2.6, 2.8, 3.0, and 3.2 GHz. Jumping from 2.4 GHz to 2.8 GHz, we thought the system seemed slightly faster, but it may have been our imagination. Jumping from 2.4 to 3.0 GHz, there was definitely a difference, but not enough to matter to most people, given the price differential. Jumping from 2.4 to 3.2 GHz, there wasn't any doubt. The system was noticeably faster, but when we checked the current price of the 3.2 GHz processor we clutched our collective chests.
i got that paragraph from http://www.hardwareguys.com/guides/perf-intel.html
.... if you want the whole article.