OP 750W would be enough to run 6970 CF, which is what you're looking at if you unlock your second card. Just google 6970 CF power supply requirements.
You should be safe with 850W even with your i7 OC. So long as the PSU isn't old and degraded.
The safest way to unlock the second card is to remove the first and perform the unlock.
Then you plug the second card back in and crossfire your unlocked 6950's, resulting in 6970 Crossfire performance.