Here's how I got xfwm4 to replace metacity in gnome.
Go to Preferences -> Advanced -> Sessions. Click on Current Session tab and then find metacity in the list. Change it's Style from restart to normal.
Then in a terminal type "killall metacity && xfwm4 &".
Of course you should replace xfwm4 with whichever WM you want to use. Although you should try xfwm4 if you havent - http://xfce.org