fixing virtual resolution only for external vga?

Hi list,

I am trying to get a projector working on a dell D 820. This machine has a 
1680x1050 resolution on its display. When I configure it with SaX2, to show a 
1024x768 or 1280x1024 resolution on the external port, I get a 1024x1024 
resolution out of it. 
Then I tried to switch through the resolutions via Ctrl+Alt+Plus. When I am at 
1024x768, then the external VGA resolution switchted to that resolution too, 
but the virtual resolution was still larger. 

Then I tried to fix the virtual Resolution via a Virtual 1024 788 line in the 
Display SubSections of the xorg.conf file. This made the external port 
working, but this also fixed the resolution on the display to 1024x768 too.

I think I need to fix the virtual resolution of the external display somehow, 
without affecting the resolution of the LCD.
Is there a way, with or without SaX?

kind regards
Sebastian