The point I was making is that just slapping in a faster processor does not realy speed up the machine a whole hell of alot because other parts are holding it back. You can slap a G4 into an old Nubus PPC machine and it will still be slow because of slow video, scsi, and low ram limits. It would not be as usefull as the same chip in a beige g3, which would not be as fast as the same chip in a B&W g3, which would not be as fast as the same chip in a real G4 machine.
The major issues with CPU upgrades is people are tempted to install a much newer OS because of the upgrade, and the slowness of the other components or hardware limitations come to light as a result.
If you upgrade the CPU and keep everything else the same you will notice speed increases depending on what you are doing, but sometimes a faster HD or more RAM might have been a better option.