I have plugged external monitor 17" to netbook and even have distortion picture, but xrandr doesn't see second screen, only default monitor. GMA500 issue own of resolution is correct now by wiki ...
Asus "T91mt" (resolution 1024x600). External monitor 17" VGA 1280x1024. Ubuntu 12.04.1. GMA500. I have solved a common gma500 issue of resolution own display, but i can't connect external monitor ...