That really depends on the graphics card. The 6 and 8 pin power connectors are used to provide power to cards that need more than the 75 W that the PCI-e slot provides.
Ideally your power supply should have a few on it already if you want to power a big graphics card. There are adapters if you really really need them but they are not an ideal solution since your power supply might not be able to provide the required power. There are 3 main types of PCI-e connectors, there are 6 pins, 8 pin, and 6+2 pin connectors which can serve as a 6 pin or an 8 pin.