Ahh, so you do plan on overclocking. That's the selling point.
You WILL want an HD5850. Particularly an Asus DirectCU edition, which is $320. This card will outperform the $400 HD5870 when overclocked, and will stay very cool. An HD5770 or HD5830 will struggle on 1920x1080, especially with the more recent games (Metro2033, etc).