When connecting to a server without a monitor by VNC, X won’t allow you to select screen resolution higher than 640×480 on next restart. This can be easily fixed:
Open /etx/X11/xorg.conf using a text editor (pico /etx/X11/xorg.conf) and add these two lines in the Monitor section of the config file:
Now reboot X or your server and next time you connect using VNC you’ll be able to select up to 1024×768.
The Monitor Section will look something like this when you have added the two lines:
Identifier "Configured Monitor"
This has only been tested on Ubuntu 8 but it should work on Debian.