I had a similar problem with a Geforce4 440 Go in my Toshiba Satellite. Most people are able to solve their problem just "finding" the right modeline, but not in my case.
After lots of research I found something to solve my problem:
There are a few bunch of cases where the actual size of the display is read incorrectly because of corrupted data (don't know weather from the display or the card itself).
Try following the steps in the link above and check is your problem is caused by this. That is, installing read-edid, obtaining the edid info from the Nvidia X Serving settings, and checking the file with parse-edid.
I hope you can solve your problem.