You don't have to choose "1900 x 1200." Choose the highest resolution that your Display Properties setup allows.
Your problem exists solely with the limitations of the graphics chip. Many people use lower resolutions than the monitor supports. Vista (and Windows 7) have improved graphics engines which would likely also improve your visual quality in case you consider buying Windows 7 OEM when released.
Make sure that you use the correct ratio which is about 1.6/1. It seems likely that 1680x1050 should be supported.
Please post back and let us know how it goes.
(If your monitor's name shows up it should be properly supported. You want "Hide Modes that this Monitor can't display" checked in the Advanced settings->. You generally have options of 800x600 up to the maximum supported.
Your NVidia drivers (through your NVidia Control Panel -> likely accessible from your Display Settings in the Control Panel) probably have three options on how to show your screen. One option is to stretch wide (not recommended), another option is to show the exact resolution (i.e. 800x600 would show black bars on all 4 sides), and the 3rd option is what you want which stretches evenly until horizontal or vertical are maxed out.
I suppose you could use the second option with "1600x1200" which would give you black bars on the left and right.
Hope this isn't confusing, again I think it possible that 1680x1050 would be supported and if so, probably your best choice.
If you have to decide, you should also look at the text to see if it gets too fuzzy (a scaling problem), but you really want the full screen at the correct ratio and maximum resolution.