I have just installed a fresh copy of the new Ubuntu 8.10.
In order to mantain all the packages installed in my old system I generated a list of the currently installed packages, so I could restore them later.
dpkg --get-selections > packages
This generates a file with all the installed packages in your system. Just copy it to a safe place, usbstick or CD.
After you have installed the fresh copy of Ubuntu you can recover what was installed in your previous system with the file you generated and the following commands:
dpkg --set-selections < packages apt-get dselect-upgrade