Start system-config-display and select the SGI 1600SW monitor in the
monitor dialog.  Then go back to the "Settings" tab and see that only
640x480 is offered in the resolution dialog.  The list needs to offer
the 1600x1024 resolution for this monitor, which is a non-standard res.

The problem is made worse when using firstboot.  If you boot the
installer with resolution=1600x1024, anaconda will create the config
file correctly.  After rebooting, firstboot will start since the
xorg.conf file is OK.  Once you get to the display section of
firstboot, it only offers 640x480 resolution, which essentially forces
you to break your configuration file.  There's no way to select
1600x1024 resolution and no way to cancel out of firstboot.