Power, temps and noise
The two GTX 670 cards draw marginally more system-wide power than the Beast™ and an extra 155W or so over a single card.
There's more hot air in the chassis with two cards, obviously, hence one reason for the higher temperatures.
The two cards' fans need to work slightly harder in keeping the temperatures in check. This is why the noise from two cards in the test system is higher than either the GTX 690 or single-card GTX 670. Putting a subjective spin on it, the noise never becomes irritating at any time.
