It depends on the rest of your system and what you are going to do with it?
Even the weaker cards support bigger resolution than your monitor!
even my gefore 6200 card can support higher resolution than my monitor can handle. viewsonic 1680 x 1050. (doesn't mean games will play well on weak gpus but very high resolutions are supported.)
Just get a good card thats in you budget. http://www.tomshardware.com/reviews/best-graphics-card,2521.html
Use this link to go to toms latest "Best Graphics Cards For The Money: January 2010 "