Here it’s a tip on how to speed up your system a bit and get ride of metacity, without loosing all the benefits of the gnome-platform.

• Install a window manager, in my case Openbox, sudo aptitude install openbox obconf
• Open your Configuration Editor, go to Applications -> System Tools -> Configuration Editor
• Go to / -> desktop -> gnome -> applications -> window_manager
• Change Default value, for example to /usr/bin/openbox
• Now restart (Ctrl-Alt-Backspace)

You could have also changed the gconf key at ~/.gconf/desktop/gnome/applications/window_manager/%gconf.xml

UPDATE: Version of openbox in ubuntu 7.04 is too old and slow, grab last version from here