When I switched to Leopard, I just popped the disk in and followed the default prompts. I didn't archive/install, but it certainly is prudent to have your hard drive backed up. It went very well and I've not had a single problem with Leopard.
I back up using Super Duper, which makes a fully bootable cloned image. IIUC, you can reformat the partition you're going to use, do a clean install of Leopard on it, and use Migration Assistant to automatically restore your users and files from the backed up disk image.