You can just do an upgrade install, and all of your stuff will remain intact. You may have some apps that aren't Leopard-compatible though, so make sure you figure out what's what before you do so. Many OS X apps have Leopard-compatible versions out by now, so you can usually just make sure you've upgraded everything to the latest versions.
Many people recommend doing an Archive and Install or an Erase and Install to wipe everything clean and start afresh to give themselves the most stable system possible. It's more time-consuming, but if you already back your system up regularly as you should, it's not a terrible chore to bring what you need from your backup to your new Leopard system.