Radeon HD 3870 X2

Ok, how does that work? I understand it's two "cores", in a sense, two vid. cards stuck together so they fit in one card slot. Making it one physical card, that holds two cards or whatever. But how do games read that card? Do they read it as two cards, or one sli?

Does that have a performance hit being two cores, or is it better than a GeForce 8800GTX Ultra (not looking at the massive price difference right now, just trying to determine better power overall).

Thanks