Mac OS X has always given you the option to upgrade your current system, or erase and install. I would assume 10.6 won't be any different. I would guess you've not been a Mac user long if you've never done a system install, so you're probably running 10.5 on a system thats less than 2 years old. You're probably fine to just install on top of what you've got which won't involve losing any of your files or settings.