Ask Max: Preparing for the Leopard upgrade
What do I need to do to be ready for my new operating system? Should everything be backed up? Will my dual G5 need any tweaking?
The Leopard upgrade is often smooth, having done it on a dozen or so systems myself I can attest to that. I would, however, recommend that you buy an external drive and clone your system onto it. Use carbon Copy Cloner to make your clone.
After you have a good clone of your computer's hard drive, you can then upgrade your system to Leopard. If you encounter any problems, format the internal hard drive and install Leopard on the empty drive. Then use the Migration Assistant -which runs when you first start up Leopard- to pull the data from the cloned system you made on the external drive. Once you have everything running smoothly, you can format that external drive and turn it into your Time Machine drive.
Hope that helps... and you are going to love Leopard.