*Skip to 'Current System' if you want to directly answer my question*

Hi guys,
I'm actually looking for a change in the entire OS, not just the desktop environment. My computer is currently a single boot Ubuntu OS, with GRUB boot-loader installed when I first used a live USB stick to install Ubuntu. I've heard that Lubuntu is very good at memory/CPU usage optimization, just what I need for a game server (yes it is a server, but I prefer graphical interface over command-line interface any day). The reason why I'm choosing to reinstall is because of avoiding the 'bloatedness' of Ubuntu.

Current system:
Ubuntu 12.04
Planned system:
Lubuntu 12.04

Main Question:
How am I supposed to back up all of my preferences, etc. to e.g. an external HDD and then restore it when I clean install another Linux system based on Ubuntu?

Thanks in advance,
sammy0025