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.
cc: self
This should be fixed in new s-c-d, I don't have the bug number handy though. Please reopen if this is still an issue for you.