Both cables are for the PCIe x16 video card. And for the PCIe 2.0 x16 for that matter. It's because bigger cards require more power. A single 8800GT, 9600GT, or 4850 etc. requires only a single 6-pin power cable because it doesn't consume very much power. Some of the bigger cards like the 9800GX2 and the GTX 280 require a lot of power and use one 6-pin connector and one 8-pin connector. As of now, the most a video card uses is a single 8-pin and 6-pin. You would need the 2 8-pins if you wanted to put two video cards together than needed the 8-pin. The 4 6-pins are for cards that require only 2 6-pins, and if you wanted to SLI them. It all depends on the card.