This is a list of the most common DEs from heavy to light:
- Unity: only for Ubuntu, quite heavy, based on Gnome
- KDE: used with Kubuntu, very windows like and a lot of bling
- Gnome-shell: shell on top of the third generation of Gnome, very simplistic and not like anything you have seen before.
- Gnome: very configurable, you can use it without effects or with a lot of crazy effects
- XFCE: quite light on system resources, but still quite customizable
- LXDE: extreme light (the L stands for light) and not very customizable
Normally I use gnome-shell. I really like the interface, but almost all users have a love or hate relationship with it. Nobody is neutral. If I want to use my laptop on battery, than I use LXDE.
For the overclocking, you will have to ask someone else or search the internet. I've never overclocked my computer (since the major speed limiting factor is the HDD anyway) but all I did was overclocking my phone.
Maybe I should look into overclocking my computer. Since I began rendering OSM maps with tilemill my CPU gets used a lot more .