The best way to set up a system, so that you can reinstall OSs as you wish is
like this:
10-12 GB partition for /
RAM/2 partition for swap, personally I only make 256 MB for swap.
rest partition for /home
This way you will keep your personal data (and even the settings like desktop
wallpaper, desktop icons, settings for all applications) after formatting the
root partition and installing a new version or even a completely different OS
there. It will obviously not work for Windows as it can't read anything
beyond it's own filesystems, but you shouldn't install Windows anywhere in
the first place ;-)
Upgrading through the updater is preferred of course, but if you broken
something in your previous installation (it happens very often when you try
to learn something new about GNU/Linux) it might not work as expected.
