If you're using the D-sub / VGA connector (or, worse, a DVI-to-VGA adapter), the driver might not be able to draw the monitor's EDID information, and therefore can't determine the supported resolutions. This is because some D-sub connectors / adapters do not support EDID pass-through.
Check the most recent /var/log/Xorg.*.log file and see if you can find any errors, warnings or any other statements about acquiring EDID information for the display, or any other warnings related to the monitor.
If the driver can't determine the appropriate HorizSync and VertRefresh values, you'll have to provide them manually by editing the (or adding a) "Monitor" section in the /etc/X11/xorg.conf file. This is the equivalent of installing a monitor driver on Windows. The correct values should be found in your monitor's manual or, if not there, from Samsung customer support (heh). Or, download the monitor's Windows driver from the Samsung support site and open the text-formatted driver file and find the frequencies in there.
After that is set correctly, restart X (by logging out and logging back in) and then use the NVIDIA X Server Settings application to select the desired resolution.
Here's an example of what the Monitor section would look like. You need to find your display's frequencies to get the right resolutions.
# HorizSync source: edid, VertRefresh source: edid
ModelName "DELL ST2410"
HorizSync 30.0 - 83.0
VertRefresh 50.0 - 76.0
If you're already using a DVI connector and the EDID is being read correctly, then I don't know what the problem is.