I have the next step up, 6870's in crossfire. I used a single 6870 for about a month and it worked great. They outperform the 6850 but not by a whole lot. The cards I have are 1GB each, you need larger if you are going over standard 1080 resolution.
I say stick with the 1GB 6850. If you can find a 6870 at the same price point.. or close I would go that route.