you will get an average of about 450.. you might as well go for an older hd 6950 instead it will be cheaper.. consume less power and also give about 400-450.. some have unlocked their 6950s to 6970 and gotten about 500kh/s...
Not sure about the x exactly all i i know is if it has the x... its better than the one without ...
refer to this link for approx. values on hashing from the different cards
https://litecoin.info/Mining_hardware_comparison#GPU