Question if the resolution option wasn't there before then how is hiding the options that the monitor dosn't support going to make that resolution sudenly apear?
The previous poster said to "uncheck" the "hide modes..." option, ie, making more options available, not hiding them. However, I don't see how this would help anyways because the native resolution should be appearing no matter what. As you say, it doesn't seem like a solution...
It does seem like a very odd sort of problem. Its not like 1680x1050 is such a rare resolution.
I don't understand your argument for returning to non-widescreen, but if it works for you, enjoy. Personally, I like having the expanded desktop space widescreen provides - its almost like 1.5 normal monitors. If you should change your mind, I would try finding a new(er) driver for your card. Since your GPU is old, perhaps the driver you're running dates from a time when widescreen was not widely available, and hence not well supported. Any nvidia driver should work on the card, though if its very recent it may need to be unlocked to allow installation on a 4 series. Obviously, very recent is not necessary as it simply increases the size of the driver with features your card is not capable of. (Then again, you could always treat yourself to an upgrade...the 6 series is very cheap nowadays)
Whatever way you choose, hope you're happy with it.
Note: you are right that running LCD's at non-native resolution permanently is not so nice. While playing a game, you can sacrifice, but you do lose quality so as a permanent solution its not good.