From Low End Mac:
To install Leopard on an "unsupported" G4 clocked under 867 MHz:
1. Reboot your Mac and hold down the Cmd-Opt-O-F keys until you get a white screen with black text. This is the Open Firmware prompt.
2. Insert the Mac OS X Leopard Install DVD.
3. Type the following lines exactly as shown below into the Open Firmware prompt. Be mindful of capitalization, spaces, zeros, etc. If the command is properly typed and understood, Open Firmware will display "ok" at the end of each line after you hit "return". What these lines do is set the CPU speed reported by Open Firmware to OS X as an 867 MHz G4 processor system. They then continue the boot from the DVD drive.
For single CPUs, use the following three lines:
dev /cpus/PowerPC,G4@0
d# 867000000 encode-int " clock-frequency" property
boot cd:,\\:tbxi
For dual CPUs, use the following five lines:
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
4. Continue the install normally.
5. This CPU setting is only in effect until the Mac reboots. Once OS X Leopard is installed and your Mac has rebooted, the proper CPU speed should once again be displayed when you select About This Mac under the Apple menu.
UPDATE: The folks at iCode have taken the information from this article and used it to create LeopardAssist, a free program that automates this process and eliminates the need to fiddle with OpenFirware on your own.
LEM Editor's note: Some readers have noted problems booting the Leopard install disc using the above commands. If so, substitute "mac-boot" (without the quote marks) for the last line in the above Open Firmware commands. Also, if your Mac locks up hard during this process, resetting the PRAM (start your Mac and immediately hold down Cmd-Opt-P-R until your Mac restarts) should bring it back to life.
http://lowendmac.com/osx/leopard/openfirmware.html
I have used this method many times, it's really easy and works.
If you get Leopard up and running on that machine, post about it. I will hook you up with a Graphics Card, gratis. I have an Nvidia 6200 in PCI form which works a treat in MOST Macs with PCI slots, but I have never tested it in a Pre-AGP Mac.
Years back I had a Blue & White and slapped a G4 ZIF from a Yikes in it. I was trying to build up a system to edit my reel with. Trying to fire up FInal Cut I ran into a little problem with my Rage 128. And thus began my Video Card hobby.
So, I still have a warm place in my heart for such a machine.