The 2 cards are pretty close and is really not worth the extra cash you'd pay for with the 760. That being said, the 760 is better in some games and the 270x is better with higher res.
It's a pretty close call, but you should go with the 270x, that card paired with the 8320 is like 2 beasts that births a baby.....which is your rig. You wouldn't need to oc