However, I have no 6-Pin PCI Express 4-pin molex cables handy. I do, however have a 4 pin ATX to 4 pin molex power cable. Can I still use the ATX cable to connect my PCIe Firewire card to the PSU or will it only function properly using a 6-Pin PCIe connector?
FYI: The PSU has connections split in two. One section for 6-pin connectors stating: CPU/PCI, and the other section for 4-pin connectors stating Peripherals/SATA.
The card should work fine without the molex cable connected. The documentation states that it is only for "reliable" power. Try it out with just the power from the PCI lane and see if that works before you go investigating extra cables.