Upgrading to Leopard?

You can do an Archive and Install.
This will rename the old System as Previous System and install the new one.
It'll retain your apps and such.

I think there is an upgrade option which will just upgrade the OS. This will keep everything. But it may also keep anything slightly dodgy (such as problems with plists etc.)

The best option is to back up everything to an external and run a clean install. Then you can migrate the necessary stuff when you need to.

 

If you want to keep all of your settings, it's best to do an 'Upgrade' install. I've done it before and it works completely.

As bartelby said, if you've got any dodgey stuff lingering around at the moment, it'll probably carry through to Leopard. If not, go ahead an do the upgrade.

You may find that some of your apps will no longer work in Leopard, but if they're still in active development they'll probably have Leopard versions available for download. If you've got any apps that play up after the upgrade, let us know and we'll help you out.