Hey, assuming your drivers work on the Linux side (there was a big problem with this on ATI a couple years ago, but most distros have long since picked up the fix)...
1. Start up X normally on the onboard monitor
2. Bring up a console
3. Figure out what the driver named your external output. It'll probably be something like VGA-0 or just VGA. Also note the name of your internal monitor (probably LVDS-0 or just LVDS).
xrandr
4. Define your preferred modeline. Example is 640x480@60Hz (ATSC timings) using negative separate sync.
xrandr --newmode "ATSC-480-60i" 11.97 640 664 736 760 480 484 492 525 Interlace -Hsync -Vsync
5. Add the new mode to the output you identified above.
xrandr --addmode VGA-0 ATSC-480-60i
6. Activate that output using that mode and cloning your onboard monitor.
xrandr --output VGA-0 --mode ATSC-480-60i --same-as LVDS-0
Now, assuming all your drivers are happy, you should have on your VGA connector a version of your onboard monitor scaled down to 640x480 and interlaced (i.e. 15kHz). The downscaling doesn't always work right. You can instead specify that it's another head of a dual monitor configuration using --left-of, --right-of, etc. You can also use any modeline that your card is capable of, and you can name it whatever you want. I don't think there's a practical limit on the number of installed modes, but if there is, it's easy to script swapping them out on the fly.
As a note, you can also do monitor flipping, etc. using the xrandr command (it means Rotate AND Resize). Handy for cocktails or if you've got a monitor installed upside-down or bouncing off a mirror and don't want to / can't (e.g. LCD) flip the yoke connectors.
Now, if all this works, you probably want it permanent. For that, you have to modify xorg.conf. I can give you a syntax example if you like.
IIRC, somebody makes a Linux distro pre-tweaked for all this. Check the forums around here.
And before you rail on me for Linux being arcane and requiring weird commands for this, consider that the only way to do it on Windows is with driver-specific registry hacking. Soft15k and friends just automate that for you
