x2. I run my MBP in clamshell via DVI-->HDMI to a 37'' LCD that runs it in 1920x1080.works fine in OSX. Doing it right now actually..
Has to be a video driver issue then. See if you can update is with the latest Nvidia driver. Although my boot camp 1.4 driver works fine with a 24" external monitor.
is it on DVI or VGA?
I use DVI as well. VGA looks worse at those high resolutions.
So you tried the laptopvideo2go drivers? If those don't work either, maybe it's a setting in the Nvidia control panel that needs adjusting? Can't you add a custom resolution in there somewhere?