I'm posting this here, so that it can save you several days of head-banging against linux. It was specifically done with Ubuntu, but it probably applies to any Debian variant running *gdm* and *gnome*.
HOWTO: Get Dual Monitors ("Dual Head") to work on a Dell Optiplex 755 running Ubuntu 8.04 with Intel Q35 Chipset
First, the bad news: you will have to hack your */etc/X11/xorg.conf* file.
Now, the good news: there's something called *xrandr* that does most of the work for you. This post walks you through it: http://www.thinkwiki.org/wiki/Xorg_RandR_1.2
The only thing that didn't work for me was getting it to run automatically. I have to run a shell script when I first log in. All it does is run the *xrandr* command with my config. My command is:
My advice, from hours of misadventure: Ignore *displayconfig-gtk* that comes with Ubuntu. It's no help. (Besides, it's deprecated, I believe.) Also, don't bother with the *gnome-display-settings* applet you get under *System > Preferences > Screen Resolution*. It will just confuse X.
If you try to use a Virtual screen size greater than 2048x2048, you will have to turn off *compiz*. If you don't, then your screen will turn white and probably not recover before your patience runs out and you manually shutdown the XServer. (This post says you can hack it to make it work, but it didn't for me: http://ubuntuforums.org/archive/index.php/t-660712.html
) Goto *System > Preferences > Appearances > Visual Effects* and select "None".
Now, just to be nice, here's my */etc/X11/xorg.conf*. If you're not running the exact same hardware as me, it might not work.
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