You have to have 2 different rails for each 4 pin molex plug
Umm no, you dont, assuming you are referring to actual +12v rails and not simply saying that you need two seperate connectors.
Fluffy - Plug in each of the white molex connectors to whatever two free ones you have in your case. Just make sure that neither of them are connected to something like a hard drive as the fluctuations from the hard drive will interfere with the card when it tries to draw power. To only have one of them plugged in, you're still starving your card for power. Obviously it's not effecting it as bad as it was beforehand, but you still need to have BOTH molex connectors plugged in.
Plug BOTH in and plug the black part into the card. Both plugs are needed since the PCI-E power has 6 ports and a normal molex only has 4.
(this part is not towards you FluffyPirate as you're obviously somewhat new to the latest hardware changes, so just ignore this part)
It's really not that hard people, it's called an adapter to change one plug style to another.