Well, CF 5850 is very good and right now, cheap too. No reason not too.
But, like Maziar said, you're resolution isn't going to need as much GPU power as if you had 1080p, at which point CF 5850s can push any game to very high settings. at 1680x1050 I would think any game at highest settings guaranteed, with FPS to spare lol. So basically it would be a little more power than you need at your resolution, but for the price, it's a great way to go and of course you can then have the option of upgrading to a bigger monitor as well.