Well currently my HD 6870 is good enough for my usage.
And likely if I do upgrade in the feature I probably go for a single card instead of Crossfire but just looking at all the possibilities.
I mostly play Skyrim, Battlefield 3, Need for Speed - The Run, Witcher 2 Assassins of Kings, Assassin's creed Revelations and other games and I don't really have any bad experience right now.
Even when I run Skyrim on Ultra settings with a screen resolution of 1920x1080 I get like 60 fps (Limited due to vsync enabled which causes Fraps to only show 60 fps). Of course this is not all the time, sometimes it drops to like 40-50 fps but thats still good enough for me. I play at my monitors native resolution most of the times which is 1920x1080.
If I do however notice a drop in performance running that setting I lower the resolution and also lower the graphic settings which most of the times works well.