Huh, why would you need to install Tiger and Leopard?

A new mac comes preinstalled with Tiger. Just install Leopard once when you get it.

Or you could just wait a couple more weeks and save yourself that hassle.

Well installing apps is just a matter of drag n drop.

Personally, I backup my entire home folder nightly, and I also backup my Applications folder. The last time I had to reinstall Tiger due to a logic board failure, all I did was to copy everything back and I have all my apps and my settings and preferences for those apps and all back in one go.

Obviously there are a couple of apps which are less straightforward, but for the most part, it's that simple.

Seriously though, I still think it's more worthwhile to just wait.

Well, the applications bit, the most important part is actually the Application Support folders that has all your app preferences and license keys stored in it. They are kept in two places:

/Users/<your user>/Library/Application Support/

/Libarry/Application Support/

Basically, since I backup my entire /Users/<my username>/ folder, the above user library folder gets backed up as well. Most of the user installed applications store their preferences and support files there, although not all. Back up those two.

Also if you have installed screensavers, check these few too:

/Library/Screen Savers/
/Library/Preference Panes/
/Library/QuickTime/ <- for QT plugins
/Library/Spotlight/ <-- for spotlight plugins

I backed up all my data to a HFS+ formatted external disk, so permissions and etc are all preserved the way it should, and I do my daily backups via rsync.

For Applications, I just copy over the apps that I often use into a separate folder in my backup disk. I only copy applications that are non-bundled with all Macs into it (meaning, I leave out stuff like Address Book, iCal, etc).

Obviously this won't work for ALL applications, stuff like Adobe Photoshop or something just won't work like that. But most OS X apps do, and that saves you a lot of hassle of opening .dmg files one after another to copy them back in one by one.

Apps are backward compatible, so don't worry about that. The only thing that's new for Leopard-only apps are applications that make use of the new frameworks and features that are new to Leopard.