Your PSU should have it, otherwise you're going to need a 2-Molex-to-6Pin-PCIe. But if I were you I'd check if your PSU can even handle the X1950XT before going out to buy anything else.
Check the power requirements on the box.
Mine says 450watt minimum and 30Amps on the 12V rail. That is huge. I'm struggling to find a power supply that will deliver that.