Power Consumption

A single Radeon HD 3870 is quite power efficient, but putting two on a board inevitably is going to draw a bit of current. It's no surprise that the X2 is not only the hottest card out of the bunch, but it also makes the most noise; there's simply a lot of heat to expel from an admittedly well designed card.

If cool and quiet are your goals, the 3870 X2 is not a good answer.