What resolutions are available?
From the EDID data of the monitor to one degree (i.e. the back-channel from the monitor to the system where the monitor provides the available display modes), then from the monitor data describing the necessary timing for the monitor (e.g. minimal, maximal frequencies, etc.) and from the available graphics memory. I would look into the corresponding log file (/var/log/Xorg.log in your case, likely) and see whether Xorg has anything to say about your monitor and your card. Check for warning (WW) and error (EE) lines. You can also try to add the resolution manually to Xorg.conf and see what happens.
