You mean 2x4 (=8) pin? You can use an 8-pin PCIe power cable, or a 6+2 pin PCIe power cable. Both are needed. The card probably won't work at all if you don't plug in both. Some graphics cards have a piezo-speaker that will shriek at you if you forget to plug in power; others will run their fans at full speed to let you know something isn't right, and I've heard of some that will show an onscreen message.
If your PSU does not already have PCIe power cables, using adapters is generally a bad idea. Not only are they messy, but a PSU without PCIe power cables probably doesn't have the +12V amperage to run a powerful video card (regardless of what its label may claim).