If you want to overclock R600 generation of graphic cards
you will have to plug the additional eight pin power connector. You might remember
this connector our pictures that we showed here.
The first six pin connector the standard one now comes on the new Power Supply Units while the second connector might find its place in the new PSUs. Anyhow ATI will bundle this cable and it will be necessary for Overdrive overclocking function.
ATI has overdrive in its R5xx and R6xx series and this tool should automatically overclock your GPU. In order to do that you need more power. That is why you need the second power connector. Remember R600 needs 230W if not even more and that is a lot of power for a graphic card.