Ubuntu :: Force Different Screen Resolution At Startup?

I recently started using Ubuntu 11.04. Everything works fine with my monitor directly connected to the computer. I get a default screen resoluton of 1680 x 1050, which is just fine.

However, when I connect the screen through a video splitter to be able to use a beamer, the screen resolution defaults to 1024x768. Moreover, Ubuntu then detects an unknown monitor and I cannot select a higher resolution.

I searched for modifications of xrandr en xorg.conf but I have to admit I don't understand what I've read. So my question remains: how can I force Ubuntu 11.04 to start with a screen resololution of 1680 x 1050 (without first connecting monitor only, that is).

I hereby add some information I retrieved with the screen connected without the splitter, so you can see what hardware is involved.

Code:
dirk@LaboFysica:~$ xrandr
Screen 0: minimum 320 x 200, current 1680 x 1050, maximum 8192 x 8192
VGA1 connected 1680x1050+0+0 (normal left inverted right x axis y axis) 474mm x 296mm
1680x1050 60.0*+
1280x1024 75.0 60.0

[Code].....