External Samsung 2443BW native resolution not listed

I have a 2007 24" iMac to which I have just added a Samsung Syncmaster 2443BW external monitor on the mini DVI interface. The Mac finds the monitor fine but the preferences panel only shows available resolutions up to 1680 x 1050 at 59.9 Hz rather than the 1920 x 1200 at 60 Hz which is native for the hardware and the same as the iMac panel. I am using the Apple DVI to VGA adapter for hookup and am running this on the analogue input (VGA) to the monitor. I have swapped the VGA cable with a known good one to no effect. Any suggestions fro how to proceed would be welcome. The extended desktop that is desired is usable at the lower resolution but the display is well below it's potential since it is running below native resolution. Thanks for any help offered
John