ATI Radeon HD5770 for $140 should give you great performance at a 1680x1050 resolution. If you got the nVidia GTX 460 768MB or HD5830 for ~$200 you might get a little bit better performance out of it, but there is probably no point getting anything higher than that.
You probably don't have the other system resources to take full advantage of the more expensive cards, but that is fine since you should be able to max out just about any game at your current resolution.
You can use this to compare two cards:http://www.anandtech.com/bench/Product/171?vs=172
Here is a good buying guide for the current cards:http://gizmodo.com/5569723/the-best-graphics-cards-at-every-price