Make:

The Leopard installer enforces the minimum requirements of 512MB RAM and a 867Mhz CPU. Under normal circumstances, if you have an older G4, you machine won't meet both of these requirements.

The RAM you can buy and upgrade. Nothing new there, but the CPU? Dylan McDermond from Low End Mac posted a hack today which shows us how to fake the CPU's clock speed. The gist is that with a few commands you can make Open Firmware report whatever clock speed you please.

Before running the install CD, you can enter the Open Firmware prompt by typing Cmd-Opt-O-F while booting (with the installer CD in). When you get the Open Firmware prompt, you can enter the following to have the firmware falsely report your CPU speed as 867Mhz and continue booting the installer CD:

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

When you reboot after completing the install, your CPU speed will be reported as normal again. Check out the link below for the full details, as well as how to adjust the reported speed for a dual-CPU system.

One thing this reminds me of is that Final Cut always refused to real-time render video transitions on iMacs and iBooks, even on models that were faster or had more memory than their Powerbook and Powermac counterparts. Makes me wonder if there is a similar Open Firmware hack to get around other software imposed limitations such as this.

Faking Out the Leopard Installer with Open Firmware - Link