I must admit to being intrigued by this "Double Lifetime Warranty". Does that mean you can kill it twice and still be covered?
Just as I thought, it's a "lifetime" warranty for you and the next owner ( each person has to register the card ).
Important - if you won't register the card in the given time you'll end up with a 1 year warranty.
As far as the XFX 6850 goes there's a black edition which costs a little more than the one I posted.
Just guessing here but it's probably clocked higher at stock. Technically that should mean more headroom when overclocking but in the end it's hardly true.
About XFX, there seem to be some problems with the quality of their RMA service as of late. No personal experience, though. As far as I know Gigabyte, Asus and EVGA have the best warranty service atm. From the 3, I've only had to RMA one Asus GPU and there weren't any problems ( but that was in Poland, not the USA ).