[Hampshire] Default X size with no screen attached

I have an aged Dell GX110 under my desk, with an integrated i180
chipset, which I normally run headless. I do have a nearby screen that
I sometimes plug in (a Dell P1110). When I boot the machine with the
screen plugged in and it starts X, it happily sets the resolution to
1280x1024 (the screen's max resolution). If I boot without the screen
plugged in, the resolution is set to 640x480.

I run x11vnc on the system so that I can access it remotely (over the
Internet, or from my laptop sitting on top of the desk that the Dell is
under). The trouble is that if I boot it without the screen attached,
the x11vnc screen is also 640x480.

Is there a way to set that default resolution higher (1024x768, or
1280x1024)? The system is using /etc/X11/xorg.conf, which I can make
available if necessary, but I was hoping that there was some generic
setting that I could make.

Any ideas?

Many thanks in advance,
Simon