I have a 6870. It is awesome, handles everything I throw at it with great graphics.
1920x1080, Plays new games like Skyrim at High settings with no issues what so ever.
You can get a nice 6870 for under $200. I have the MSI HD Radeon 6870 Twin Frozr II. Stays cool, it's very quiet, and I have not had any problems with it.
By looking at the charts and other people's posts, the Sapphire Toxic 6870 edition is also a very good choice.
You could always go with a GTX 560 also. (but I tend to stick with AMD so I don't know much about them other than info from benchmarks)