formatting HD, how to backup?

I'm just about to install Leopard, CS3 and XP on my 2.0 mbp, so I'd like to format my harddrive and zero data to start fresh. Most all my media/work files are already backed up to my external HD, how should I go about backing up the user/program/system preferences? Is it possible to selectively backup without getting too technical? Most my programs either have discs or are available online, do they need to be backed up too? Thanks.

what I do is I keep installers/dmg's of everything I have installed on my computer....
when I need to backup, I just backup that one folder

where can I find that one folder then? Sorry I've just never had to reboot an osx platform before (go figure).

ok i've backed up all the necessary files and settings, now i can't seem to find instructions on how to reformat the internal harddrive. can anyone help?