With 1680x1050 you don't absolutely need anything more powerful than a GTX 460 or ATI 5830. Both of those would do very well at that resolution with just about any game you want to play.
If you intend on a monitor upgrade anytime soon, I'd go with a faster card. Or, if you want to buy one video card to last you for the next couple years, you can certainly go faster.
But otherwise, the GTX 460 1GB would be my preference, just because the 1GB of RAM is helpful with a few game titles, and may be more helpful in the future as textures get more detailed/higher res.
You can also look at the new ATI 6850 in the same price point for equivalent performance.