Articles are starting to appear over the past week concerning upgrading to Leopard OS X 10.5. Different users have different needs when performing a major upgrade, even updates for that matter, so here's my advice.
No matter what type of installation you plan on performing, back up all of your data first. This is a no-brainer. Get a copy of SuperDuper! and use it. It's the best $28.00 you'll ever spend to keep your data backed up and secure. You'll need an external hard drive to back up the data to. An external Firewire or USB 2 drive are your best options.
For a basic user, one that simply surfs the web and emails, performing the basic upgrade should suffice. However, with any major upgrade such as the impending release of Leopard, performing an Archive and Install is probably the better option.
Archive and Install copies many user settings and applications and to a newly installed System Folder and leaves a Previous System on your hard drive. Archive and Install takes longer to perform, but is more thorough than a Basic Upgrade.
For power users, and you should know who you are, the only option you should consider is Erase and Install. This is the most time consuming method, but it is also the safest route.
Based on my experience with the creative community, video editors and graphic designers, performing an Erase and Install provides the cleanest drive setup with the fewest headaches after the procedure.
Some extra noteworthy things to do prior to upgrading. Disconnect ALL external devices from your computer BEFORE installing Leopard. Boot from the installer DVD and run the Disk Utility application found in the installer menu. Click on the First Aid tab, select your startup disk and click the Repair Disk button. Make sure that your computer firmware is up to date. Make sure that all of your external devices are synced one last time before upgrading; this includes iPods, phones, PDAs. Make sure that all of your applications and plug-ins are ready for Leopard.
If you're upgrading a Power Mac model, or another model that provides easy access to the internal hard drive, consider purchasing a new hard drive to replace the previous system drive. This should be considered if your current drive is around three years old or older. All of your previous data will be preserved on the old drive.
One more note for those Power Users. For those users that have extensive collections of plug-ins for applications, creating a clone drive AFTER installing Leopard is always a good idea. I've spent the better part of a day, 13-14 hours, installing applications and plug-ins on one computer multiple times. Purchase an extra drive and clone your system drive after your initial installation is complete. If your system drive fails, you'll only be down for about 20 minutes. That's about how long it takes to swap a drive out of a Tower model Mac.
Good luck and enjoy Leopard.
Erik