According to Apple, the minimum system requirements for Mac OS X Leopard 10.5 is a Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor; 512MB of memory; and 9GB of available disk space.

On PowerPC G4 systems with a processor upgrade card, the processor requirement becomes a roadblock when installing Leopard. This is because systems with a processor upgrade card usually need software to tell the Operating System the correct CPU speed; most often, the CPU speed shows up as 0MHz.

To install Leopard on these system, we need to let the systems know about the CPU speed before installation starts. We can do this using Open Firmware. Before starting the installation process, boot into Open Firmware and manually set the CPU speed. Afterward, start the installer and begin the installation process. Note that whenever the system reboot, the CPU speed will resets to the default system value (0MHz or the speed of your processors). On system with more than one processor, remember to set all the processors speed above the minimum 867MHz requirement.

1. Boot to Open Firmware (Cmd-Option-O-F)

You can enter Open Firmware by pressing the key combination (Cmd-Option-O-F) just as you power on a Macintosh. The Cmd key is the one with the Apple logo, and the Option key is the same as the Alt key.

2. Set CPU Speed Above 867MHz

Now we can set the CPU speed. The following commands set the CPU speed to 999Mhz and starts the multi-boot program.

3. Select boot device with Leopard Installer and start installation.

At this point, the Leopard DVD should be in the DVD drive. Simply select the device with the Leopard Installer and follow the installation instruction. When the installation process finish, check the processor speed in the “About This Mac” menu and see what shows up there.