maybe try plugging the monitor into a different connection on the card? if the monitor has hdmi try plugging it in there and see if it detects it any differently? also here is a link to a thread where a guy had the same issue as you http://www.pchelpforum.com/xf/threads/generic-non-pnp-monitor.82182/
and more specifically the solution he used
Download Phoenix.zip and extract it to a folder.
Run it and click the registry toolbar button. (This did not work on my Windows 7 , fortunately I had dual boot XP and did it there)
Click the pencil toolbar button for edit mode.
Change the EDID settings to reflect you display's true capabilities
Pay special attention to the Standard Timings tab, enable the one which shows your desired resolution.
Save the EDID to a .DAT file.
Download moninfo.exe and install it. (Started working again on Win 7 here)
Open the saved .DAT with MonInfo.
If MonInfo reads in an displays the settings the way you configured them in Phoenix then...
Select Create INF... from the file menu and save it to the working folder.
Open Start, type devmgmt.msc and press Enter.
Select the monitor you want to change, right click and select Update Driver Software
Use the tried and true "Have Disk" method to select the INF and install it.
Reboot and enjoy your new resolution!