Don't overclock your CPU with Mac OS X!
I had a terrible time installing OS X on my 7500 with a G3 upgrade card. I had strange random crashes during the installation, strange random error messages during the installation. If the installation did happen to complete, OS X would crash randomly or just stop booting.
The problem was that I had overclocked my XLR8 G3 upgrade card. The XLR8 upgrade card has switches that can change the CPU clock speed by very small increments. This allows you to set the fastest possible CPU speed, but if you set it too high the machine will crash or become unstable. I had overclocked the G3 rated at 300 up to 333 Mhz. It worked great with Mac OS 9.04, but that high speed did not work with Mac OS X. Amazingly, I had to lower the G3 speed down to 266 for it to work reliably with OS X.
The explanation is that OS X uses CPU instructions that are never used by Mac OS 8 or 9, such as for protected memory. The PC version of the Linux installation guide gives the same warning for the same reason.
This may have nothing to do with the article in the System section about enabling the backside cache in older Macs upgraded with a G3. All these problems occurred before I enabled the backside cache.
Mac OS X Hints