I would recommend the use of the metacity compositor. It's lightweight, effective, and generally used with Unity-2D.
To enable the metacity compositor, go to your terminal and paste:
gconftool-2 -s '/apps/metacity/general/compositing_manager' --type bool true
To disable:
gconftool-2 -s '/apps/metacity/general/compositing_manager' --type bool false
You may have to log out and log back in to see a difference. Hope this helps!
To make the above process easier, the latest Ubuntu Tweak (0.7.1) can now enable and tweak metacity compositor settings.
See this article for more information. http://www.webupd8.org/2012/05/ubuntu-tweak-071-available-for-download.html