I would strongly suggest getting a single, more powerful card. Yes, it may not have as much raw power as two of the lower level cards... but in every other way, it's better.
There are less games that support crossfire/SLI.
1 card adds less heat to your system.
Microstuttering. While this may not be noticeable to some people, if you notice the framerate improvement between 2 6850s and 1 6970, there's a good chance you could find the fast-slow frame rendering inherent to multiple graphics cards.
For me, the trade offs aren't worth the improvements. Whether they are for you is something you have to decide for yourself.
As for which brand to go with, I like EVGA, but I go with NVidea cards. From what I've seen in research and my own performance, EVGA gives good, stable, and balanced performance. If you want something fancier or with a prettier looking card (though I can't really understand why that matters to anyone), another brand might be better. Check reviews before you purchase.