As it is, the 4870 X2 is the fastest available single video card on the market today bar none. With the increased efficiency in power that the 4870 requires compared to the GTX260, the HD 4870 X2 delivers great performance without requiring four 6-pin power connectors as two GTX 260s would require. The video card market is constantly in a state of flux with cards being released, but rest assured, two 4870X2 cards would offer the highest performance bar none on the market today. Palit's version of the ATI HD 4870 X2 is an excellent card, easily besting the GeForce GTX280 from NVIDIA in every test. In an apple to apples comparison, the 4870 X2 is nearly 10-20% faster across the board. Of course NVIDIA would argue that the 4870 X2 should be compared to two GTX 260s due to pricing changes.

http://www.gamepyre.com/hardwared.html?aid=1007&p=1