Thanks to this article at lowendmac.com, you can install Mac OS X 10.5 Leopard (the most modern and last Mac OS to support PowerPC) on macs that don’t meet the 867 MHz CPU requirement the installer checks for. This has been quite handy for my G4 Sawtooth, Mystic and Tangent machines.
Insert the Leopard install DVD. Reboot into Open Firmware by holding down Cmd-Opt-O-F following the tone. At the prompt, type the following exactly:
Single Processor:
dev /cpus/PowerPC,G4@0
d# 867000000 encode-int " clock-frequency" property
boot cd:,\:tbxi
Dual Processors:
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
Bingo. The system reboots and passes the CPU speed check the installer performs. This trick will last until the next reboot, when the property returns to normal.