Setting up X for external monitor(s)

[Jesse Sheidlower]

I have XFree86 4.3 running on an IBM ThinkPad X23 laptop; the
screen has a maximum resolution of 1024x768. If I plug in an
external monitor with a larger screen, and then hit F7 to get
the monitor on, it will still show up as 1024x768, just
larger.

What sections do I need to set up in my XF86Config-4 file to
enable different resolutions for an external monitor? I'm not
clear on the differences between Section "Monitor" and the
Section "Screen" (where resolutions appear to be set). And
specifically, how do I then switch from monitor to monitor, so
that I'm using a particular one for the external monitor, with
bigger resolution, and a different one for the laptop screen,
with a smaller resolution (i.e. something like "hit F7")? What
if I had the possibility of several different external
monitors, with different resolutions?

I'm using Gnome2 if that matters, on FreeBSD 4.8. The computer
uses the ATI Radeon Mobility card.

Thanks.

Jesse Sheidlower