How to configure X server to work headless as well with any monitor connected?


I have managed to start Xserver without any screen attached to server.

First: I have created xorg file with nvidia-xconfig tool.
Second: I have set Option “AllowEmptyInitialConfiguration”

Now I can connect to machine with VNC but I can not set a resolution bigger than 640x480

[ 9.672] (II) NVIDIA(0): Virtual screen size determined to be 640 x 480
[ 9.697] (WW) NVIDIA(0): Unable to get display device for DPI computation.

Do you know how can I set a bigger resolution for virtual screen?