Big desktop mode enabled by amdcccle (which, by the way, blew my mind. I didn't have to touch my configuration files to enable it. Threw in the VGA cable, aticonfigged it, enabled it and away we were).
My modes are:
Now, if I click on the desktop near the right hand side of the VGA monitor (the 1680x1050) or drag a window to be very large and include, say, the last 500 pixels on the right side, or even make a desktop (nautilus) selection that big, the X server will crash. It's happening pretty reliably.