The second display works fine if you attach the vga adaptor before booting the laptop. I had some presentations with beamers at 1400x1050 resolution; works like a charm.
If you connect the external vga adaptor later, you can change the display used for output with Fn+F5. If you do this, the upper third (640x480), the upper fourth (800x600) or the upper fifth (1024x768) of the screen is scrambled and not usable. The rest of the screen is moved down, so that you won't see your kicker (task bar). Perhaps that wouldn't be soo bad, but the mouse cursor uses the whole screen, so that you've got to click 50px above the button to activate it. Very annoying.
The screen stays scrambled even if you are back to "LCD only"-mode. Switch to a console and switch back to X, and it's nice again. No need for restarting X.
When using the "auto" BIOS mode and the second device attached while booting, only the second display is used.
The usable solution comes when using the "both" BIOS mode. The screen contents are on both screens, but the borders (approx 30px) are cut off. You can switch the resolution with krandrtray or whatever, but even at 1024x768 the borders are missing. But this mode can be used for presentations; I already did it.
If you are in "both" mode, DON'T use the Fn+F5 switch.
A notable thing is that you can drive your LCD at 1400x1050, for the second display it is downscaled. But that doesn't look nice.
There are two tools written for the i855 chipsets which allow switching the second crt on and off:
Connect the external vga before booting
This is a universal tool for all chipsets based on the i810 one, and the i855 belongs to that group. With the simple call
i810switch crt on
the second display gets active and displays even the 1400x1050 resolution (if the monitor supports it). Now it's pretty unusable as the picture flickers really bad. Try the next tool:
This program does the same as i810switch, but only for 855GM chipsets and with a different technique. Do a simple
./i855crt on 1024x768@85
and the second display works fine. 60Hz and 85Hz seem to be supported in every resolution, 70Hz not. Alas the 1400x1050 resolution does not work...
But with Ctrl+Alt+(plus|minus on
keypad) you can shrink the resolution, but have the
virtual screen still on 1400x1050, meaning that you can scroll
your screen. The scrolling does not work on the second display;
the scroll position of the time the script has been executed is
kept.