What's Better, 2 x GTX460 or 1 GTX560ti?

How well do video cards work together? i have a gtx460 and i wonder if i get another one, would two of them working together be almost as fast as the gtx580 or close?|||Two GTX 460 with the cores clocked at stock speeds get you performance that falls between a GTX 570 and 580. You'd have to overclock the cards if you want GTX 580 performance. Half of these GTX 460 cards are pre-overclocked anyway.