Try right clicking on the (non-Steam) shortcut for the game (or for any version on the DCApp.exe file in the ..\Divinity Dragon Commander\Shipping\ folder) and selecting 'Run as administrator', and see if that gets any resolutions to show up in the options. You can also right click, select Properties and in the Compatibility tab try some compatibility modes (and in Win 7 maybe disable desktop composition).
Did you try switching display modes? Try deleting or renaming the graphicSettings.lsx file to get the game to recreate it the next time it is started.
I assume your desktop is at something higher that 800x600... You can try changing your desktop resolution. One person had a particular resolution missing from the graphics options list, but changing their desktop resolution to that got it to show up in the game, and they then changed their desktop resolution back.
