If the 6950 is within your budget, it is the performance king in that price range, especially if you are willing to unlock it and as you noted, is a much better deal than the 580. Go for it!
The second lane is limited to x4 speed, but crossfire does a bit of load balancing between the cards, and this isn't as big a hit as you might expect. I've read two reviews which showed you'd lose a little performance, though with the 6950 cards being a lot faster the hit might be a bit more noticeable. No-one can say definitively what the result will be.
Chances are though, when you do get to the point that you want to get a second card to crossfire, you'll be facing the exact same decision you are now: there'll be single cards that offer more than double the power at a price you can afford, and you'll be thinking, if I get a second one to crossfire now, when it's next time to upgrade I'll have to sell them both...