You cant look that far ahead really. At some point you will physically need more power or features to play the latest games and even if you got a 5870 now i would think that in 3-4 years there would already be a card out that was better or more feature packed for a decent price.
DX11 has yet to take off properly, but if you want to be as up to date as possable then a DX11 card is what you need.
If however you dont ned the latest features and just want to play games to a decent level then by al means look at a DX10.1 card. I would recomend that you get something with more raw power though at least a 4870-4890. Thats if your PSU is up to it.
I am now on the fence of either liking DX11 or hating it. It's sweet to look at on the Heaven benchmark, but for Dirt2 DX11 was put in at the very last moment so it's rather buggy. I actually own a 5970 BE from XfX and honestly half the time I have noticed my two 5870's I had actually run every game better. But I am sure that's a driver issue(hopefully). But DX11 is still really new. I wouldn't see to many games actually taking advantage in DX11 until Nvidia comes out with their DX11 card.
Crossfired HD5870s should be better than an HD5970. The HD5970 cores have 80 texture units like the HD5870 but they are clocked the same as an HD5850 so it's somewhere in between crossfiring either of them.
Judging DX11 based on the first game that supports it is rather silly imo. Try the new stalker game, it supposedly uses DX11 to better effect. It's not out in the US yet but I believe you can DL a demo/benchmarker for it.