Ah, the 600 is four rails, sorry.
Bah, the mechanical spec dwaing states tehPCIx draws power from 12v4... there is no 12V4!!!
That's because it's drawing power from subspace
, in the great and good world of Nvidia
it's possible for the card to work in 2D mode but crash when asked to go 3D or SLI if there is not enough power available.