Cards like HD6750 requires a separate power connector which every good branded PSU comes with and you will get a pci-e/molex power converter with the card box as well - just use the 6 pin pci-e power cable and fit it into the appropriate pci-e power connector of the HD6750
HD6670 consumes less power than HD6750 and the pci-e slot on the mobo is enough to power this up - so you don't have to use any additional power cable with cards like these