Although having one more powerful graphics card is usually the best way to go, two 6850s in crossfire should be significantly better performing than one 6970 and possibly a bit cheaper.
Don't want to put you off, but I've never had crossfire work properly with my 6850s. However the VAST majority of people get these cards working absolutely gorgeously in crossfire, so long as you have a good processor, power supply and motherboard.
For that reason I would go with the 6850s in crossfire.
A single card is always better than a dual at any time in terms of 'convenience'. Because crossfire support for gaming depends a lot on drivers compared to single card.
Moreover it is been repeatedly mentioned across the web that CF 6850s may have microstuttering problems. here
Unless you go for an eyefinity set-up, I would suggest you a GTX570. The explanation is simple: Most games support Nvidia better than AMD. Though the difference is very minimal in terms of fps, the game crashes & bugs due the card/drivers makes the difference.
And btw, Saphire Toxic edition is the way for AMD many say...
I have to disagree with getting one 6970 or one GTX 570. Of course you can face some problems with crossfire, but the fact is lots of games support crossfire and even those that don't usually benefit from increased performance.
Issues are always being ironed out with driver updates and CAPs, you get very good performance with 6850s in crossfire, a noticeable increase if you came from a single 6970 or GTX 570. The fact that 2 6850s is probably cheaper than a 6970 or 570 makes the 6850s in crossfire and even better choice.
The 6970 and 570s are great cards without a doubt, but the fact that you can get better performance on the whole with 2 cards that total a cost that is cheaper makes the 6850s such a sweet spot for both performance and price.
As for which brand to go for I'm not too up together on which brands are the best but you should definitely check whether the card's voltage can be changed if you plan to overclock because I've got a pair of Gigabyte HD6850 OC editions, and despite their name, they don't overclock all that well because they have a cheap and nasty voltage controller that doesn't allow you to increase the voltage meaning you can't unlock the true overclocking potential of the cards.
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.
For me, the trade offs aren't worth the improvements
Huh what? Two is better than one period. You get 90% scaling with two over one - Get the 6850's but if you have cash flow get the 6970 and CF that later on - simple. And realistically I wouldn't even buy any card right now. The new lower to mid-range 7 series cards are coming this month or next. The mid-range 7 series cards are rumored to beat the high-end 6 series cards with lower power draw, clocked higher, etc... Anyone that's saying 1>2 is just hating and blatantly disregarding facts.
thanks for all the quick feedbacks, all good suggestions, seems to me that 2 6850 is better than 1 6970 and benchmarks supports it. however, might be potential tweaking involve to make it work.
the bigger picture is that i'm trying to get my budget under $1000 for the system, my 13 yr old son and i will be building this system together, it will be a good exercise for him to build a system from the ground up.
If you are learning I will suggest 1 card then add in another later. Perhaps start with a 6870 or 6950 1 GB and add another if you feel the need. Learn with one, as for all this corssfire SLI advice I would take it with a grain of salt there are not articles all over this website and others because an anoying problem doesn't exist.
Oh by the way, the purpose of this besides having a father and son time building the PC, is to make this a gaming PC and also to brag against his other friend who's also getting a gaming PC, not building it, but buying preconfigured. i wan this to be good, but not break the bank.
Make sure that the Gigabyte 6850 isn't Gigabyte's factory OCed one, you can't change the voltage which makes overclocking past a certain speed impossible. Ironically the non factory overclocked version does allow you to change voltage and so it overclocks better.
The equivalent PC prebuilt would cost hundreds more, so it's a win win situation. The system will be a great gaming PC and brag-worthy