Install Mac OS X Leopard on a G4 800mhz Quicksilver

If you have an old G4 sitting around that’s at the 800mhz mark, you probably should try installing Leopard, because most people agree it actually runs FASTER than Mac OS X 10.4 Tiger. Weird, huh? I guess they’ve optimized the code pretty well.
However, when you try to install the operating system, you are warned that Leopard cannot be installed on your G4. There are a few reasons for this:
1) Leopard requires 512MB of RAM – you have RAM, right?
2) Leopard requires over 867mhz processor

We can fix number 1 by simply getting more RAM. I find Craigslist to be of great use here. Number two is a bit more difficult as G4 processor upgrades are ridiculously expensive once you consider the cost of a Mac Mini, and also requires a bit of tech savvy under the hood as you’d be swapping CPUs.
Not to worry, though. Here’s a way to convince OpenFirmware that your CPU is 867mhz, and allow the installer to boot, install, and get you off and running:
Boot into Open Firmware, I have covered this extensively here:
Once in OpenFirmware, issue the following commands (for single CPU):
dev /cpus/PowerPC,G4@0
d# 867000000 encode-int " clock-frequency" property
boot cd:,\\:tbxi

For dual CPU:
dev /cpus/PowerPC,G4@0
d# 867000000 encode-int " clock-frequency" property
dev /cpus/PowerPC,G4@1
d# 867000000 encode-int " clock-frequency" property
boot cd:,\\:tbxi

Note that all we are doing is over-writing the CPU clock-frequency (speed) property for each CPU installed, at boot time.
Also, if you need to boot another device, try:
printenv boot-device
This will return a list of boot devices to use when booting the installer. I used this in order to boot a Firewire device that had had a disc image (DMG) restored to it, making things a bit easy and faster.

Good luck!