If you are dealing with two cards, its normal crossfire, and you would need a second 4670 from any brand, not a second HIS card specifically.
With 2 4670s it wont boost your gaming a ton, you would probably be better off selling of the other one and getting a single higher end card instead if you need more performance, it wont help at all for video editing, not all apps support crossfire and most things that use GPU acceleration dont.