6850's in crossfire are going to provide you with higher performance than the GTX 570, but do you need that much power? At that resolution a 570 or 6850's in in crossfire would both be plenty. I am a fan of using two cards to get more performance for your money (above a certain performance point). However, be aware that there is the potential for hangups and/or issues when using multiple cards. A lot of the people that provide advice here will recommend going for the most powerful single card in your budget/performance range to simplify.
If you do go with a GTX 570 you have the option of getting a second one down the road to further improve your performance. If you go with the 6850's, you don't have an upgrade path for your graphics without starting over (they are limited to 2 cards in crossfire- not a big deal since not a lot of folks go 3+ cards).
It's up to you to determine what's important to you: better value with the potential of having to deal with multi-card problems or getting a little bit less of a value and having the simplicity/ease of just one card (+ having a great upgrade path if you want to double up on that 570). Also, you will need a bigger monitor for that monster rig you're building