None of these are exactly "Modern Day Gaming" Cards and they will struggle with the games listed.
9 Series: Bleeding Edge Components, Max out All New Games, Multi Monitors etc..
8 Series: Performance parts, Should be able to play new games on high with good frames
7: Entry-Level Gaming, Can run old games on high detail, and can run new games with decent frames, but without eye-candy.
6 and below: Multimedia Oriented Cards. Not suitable for anything but light gaming.
Now, in saying this, the top end game evolves as the graphic card does, so a 6850 and a 7850 will not even be comparable.
For your purpose, I would recommend at least a 560/6870/7850 for those games, but if i had to choose one of those cards, 4870.
So if I figured out to buy the 6870 , Will that be a decent graphics card for modern gaming ??