XFCE: Change window manager to metacity

Hello!

Due to personal preference, on my old machine, which uses XFCE as desktop, I have changed the default window manager to metacity.

Now I want to do the same on my new machine, but unfortunately I have both forgotten what I had done exactly and cannot find a satisfying solution on the web either. WHAT I know is that i have edited or maybe even created some global configuration file, which says something like "STARTWINDOWMANAGER=metacity". However, on the old machine I have already grep'd the whole of /etc/ for promising strings, with no reasonable result whatsoever. So please, anyone here who knows what to do?

Thanks!
Lennex