It depends on what you are using the computer for. For instance, running distributed projects like SETI, cache makes a huge difference. I have a P4 1.4 GHz with 1 MB L2 cache, a P4 3 GHz with 512K cache and an Athlon running 2.1 GHz with 256 MB L2 cache. The P4 1.4 GHz completes a work unit in about 2.6 hours, the P4 3 GHz takes about 2.8 hours and the Athlon is just shy of 3 hours. That being said, for everyday use (office apps, web browsing, etc), all are about the same in look and feel.
-RADAR