I hate GNOME, really, I can't stand it. But my GeForce2 MX400 doesn't stand KDE4, and she has the last word.
I discovered a few days ago that she likes Compiz, so I set it up for her. After it was al OK I wanted GNOME to know that the default window manager has changed, but the GNOME's System→Preferences→Appearance→Visual Effects tab is full of shit, and it doesn't do the right thing, so I googled a little, with no results.
So I decided to find out for my self, and after a lot of "find" and "grep" and "vim" I tried gconf-editor. I hate the gconf stuff, it doesn't makes any sense, it's like the windows registry but partially, I don't get it. But there it was, not in:
(possibly the most logical place), but in:
There it was, "widnowmanager". Just changed it and now GNOME doesn't starts metacity, but compiz instead.