How do I replace Metacity with another Window Manager?

The easiest way to do this is to install a window manager supported by ubuntu. XFCE is the official one. The official title is xubuntu. You can install it by doing

sudo aptitude install xubuntu-desktop

(You can use apt-get instead of aptitude but some will say aptitude is better for meta-packages since it makes it easier to uninstall)

Once installed, log out and then choose the sessions button and choose XFCE (I think that's it's name in the sessions menu) and then log in. You will still have gnome installed.

Jim Hutchinson --> XFCE isn't a window manager, but an entire desktop like Gnome or KDE. XFCE window manager is XFM, Gnome window manager is MetaCity and KDE window manager is KWin.

Wersdaluv --> You can try to remove MetaCity from the Gnome Session Manager (System --> Preferences --> Session, in the second tab), then "kill -9 metacity" in a terminal and launch the new window manager, then return to Gnome Session Manager and save the current session.

But one thing you must know, MetaCity is a LIGHT window manager, you aren't going to get a better performance in gnome by changing the WM. If you really want to go light, change the entire desktop as Jim said.

@Raul - thanks for the clarification but I didn't think he would get any kind of "light" environment without moving to something like xubuntu. I probably should have been more clear. I'm still new at this.

So you think replacing metacity with xfwm is a bad idea? I managed to do it by following this http://doc.gwos.org/index.php/Xfwm4 .

Now, I am having fun with XFwm's compositor that works with my laptop (my laptop only has a generic video card), but I really cannot compare the difference of the speed between XFwm and metacity because my XFwm runs slower since it is running a compositor.

What Window Manager do you suggest to replace Metacity and why?

I haven't said it's a bad idea, but it's pointless. Replacing the window manager in gnome isn't going to give you a lighter system because the main weight is in the desktop itself, not in the window manager.

Probably you can't even notice the difference between Metacity and XFWM when you are using Gnome (in terms of speed, responsiveness or resource utilization).

If you really want to go for a LIGHT system you must try to do what Jim said, changing the whole Gnome desktop for XFCE (or whatever you like, icewm is very light too).

Suggestions to replace Metacity? Well, there is a "Sawfish" that is very similar to Metacity and was the default in Gnome some time ago. But i wouldn't change the window manager without a good reason (conflict with something you NEED or something alike, and that's not the case as far as I know).