Minimal configuration for virtual desktop resolution

[Nikos Platis]

I am using a netbook with Intel 945 graphics adapter and native screen
resolution 1024x600. My system is Arch Linux (xorg-server 1.10.1) with
KDE 4.6.3.

I am frequently connecting the VGA output of the netbook to an
external projector with resolution 1024x768. When this happens, I am
able to set the two displays in clone mode using KDE's krandr utility,
but the internal display retains its 1024x600 resolution so some
pixels at the bottom disappear.

So the question is, how will I be able to setup the internal display
to use a virtual resolution of 1024x768 in this case (and its native
resolution otherwise)? It wouldn't be bad to be able to select (on
demand) other larger virtual resolutions as well.
I would like to keep manual configuration to the minimum, since
everything else works very well.

Thanks in advance,
Nikos Platis