UPDATE: With Ubuntu 12.10, you can just go to Displays and turn off Sticky Edges.
So I plugged in the VGA cable of my new hp monitor, and guess what, the system didn’t recognize the newcomer. In most cases it should just detect the display, but it didn’t work in this case. The solution was to go to the X Server Display Configuration in Nvidia X Server Settings and click Detect Displays.
The next problem was when moving the mouse from one display to another, the pointer would slow down and wait for a few moments at the Unity launcher. This seems to be by design, but it’s a nuisance. The fix is going to /apps/compiz-1/plugins/unityshell/screen0/options in gconf-editor and changing the values of overcome_pressure and stop_velocity to 1 and 20 respectively. Later learned that you can edit these settings simply using the Ubuntu Unity plugin in CCSM.