I'm assuming you have an HD 5850 as the GPU. If that's right, the card has a port that needs both the 6 pin and 6+2 pin attached to it (leave the +2 part dangling). The card should come with two adapters that you can use. The adaptars take two molex (4 pin) connectors (what you use to power the SATA devices like the HDD and opticals) and convert them to a single PCIe connector (6 pin).
EDIT: If your card didn't come with the adapters, here's what you need: molex to 6 pin adapter