If you're using some of the later drivers (with the new control panel) there is an option where you can have your HDTV treated like a monitor instead of an HDTV. Once you check/uncheck that option you should be able to change the res, etc. I can't recall the exact name of it at the moment but I think it's under the custom timing area. Once you get there, I think it'll clear.
I just disabled the treat digital display as HDTV option and then it let me select the same sort of scaling options on the Westy 37" HD monitor as it does on the Dell 24" LCD. Edit 3: Right click the display device in the nvidia control panel to get access to those options (or use the device adjustments option).
The above is what I said in another thread. The option is in a different area with the new control panel though.