To change the browser that fires up when you click on links in various GNOME applications, just change the default browser in the GNOME control center (you can bring up the panel in question directly by running gnome-default-applications-properties). Enter something like opera "%s" or mozilla "%s" in the Custom Web Browser box.
GNOME 1.4 has a capplet for changing window manager. In GNOME 2.x, you can change it
2 pages link to GnomeNotes: