Replace metacity with openbox in Unity2d

Tested with Ubuntu 11.10
Below, there are few steps to tweak Default Ubuntu's Unity2D environment little lighter. This will give some speed up, at a cost of simplifying desktop's visual style.
Window manager is an application responsible for window decoration drawing, e.g. buttons, title, borders etc.
Openbox is a small, fast, lightweight window manager.
Metacity is gnome's default window manager (nice, yet sometimes choppy and slow).
Unity 2d -- is lightweight alternative for Ubuntu's default desktop environment.

Preconditions:
Unity 2d installed.
Steps:

1. Install openbox:

   sudo apt-get install openbox

2. Optionally install additional packages (in my case these were installed as dependencies):

   sudo apt-get install obconf openbox-themes

3. Edit /usr/share/gnome-session/sessions/2d-ubuntu.session file and replace:

   DefaultProvider-windowmanager=metacity

   with

   DefaultProvider-windowmanager=openbox

4. Logout, login (with Unity 2d session).
5. Run obconf, choose some nice theme (my was found on box-look.org and changed a little) and configure buttons position.

Enjoy!