The problem is that I now want to use that as my primary display, and my Dell monitor as my secondary display off my D620 laptop with the Intel 945GM display. That means I’ll have a primary display DVI at 1920×1600 and my secondary VGA at 1680×1050. Unfortunately for whatever reason, the Intel 945GM lists the VGA first in xrandr, which is roughly the way the server sees it. This causes Gnome to choose that display as the primary place it shows the login screen, shows the menus, and so forth.
My current solution is to login and then run this script:
xrandr --output TMDS-1 --off
xrandr --output VGA --off
xrandr --output TMDS-1 --auto
xrandr --output VGA --auto
xrandr --output VGA --pos 1920x0
This turns off both displays, and turns them both back on. This causes them to overlap at position 0×0 (meaning you’ll see the displays mirrored). Then it slides the VGA display to the right, which is what I want. I’m not sure if it’d work the other way.