I created a modeline for the 1680x1050 LCD panel on my Dell 8500. I did it by hand, respecting the constraints in the NVIDIA readme, and it works well. Here it is:
Modeline "1680x1050" 115 1680 1712 1736 1752 1050 1053 1056 1093
You may also want to tell the NVIDIA driver to mantain aspect ration when scaling lower resolutions using the following option on the device section:
Option "FlatPanelProperties" "Scaling=aspect-scaled"
BTW, I thought refresh rate did not apply to LCD panels? I did the math for a "60Hz" vertical refresh to make X happy but I don't think it makes any diference, does it?