Do you currently have a 6850? If so, I'd recommend just upgrading to a single more powerful card like the R9 270x for better performance to crossfired 6850s.
You will have lower power consumption aswell. You can always go for a stronger card than the R9 270x but it was just for reference. The 6850 will probably come with a Molex to 6 pin adapter so you should be fine.
Do you currently have a 6850? If so, I'd recommend just upgrading to a single more powerful card like the R9 270x for better performance to crossfired 6850s.
You will have lower power consumption aswell. You can always go for a stronger card than the R9 270x but it was just for reference. The 6850 will probably come with a Molex to 6 pin adapter so you should be fine.