A quick google suggests each card uses 136W (11A) and each card requires 2 x 6 pin power connectors.
If Nvidia are like ATI then the SLI connectors will be in the box.
I would get 1 and see what that's like first. Unless you run a high resolution 1 might be enough but it depends on the games you play. 2 cards would be good for folding@home though.