std clocked 7750 do not come with power connection but OC cards do. Most likely it is because most people are using motherboards with version 2.0 slots which are still limited to 75 watts! It is called backwards compatibility.
SO, AMD is making the 7770 backwards compatible by adding the 6-pin pci-e slot. Then, would it be safe to assume that the 7770 can run w/o 6-pin, in a 3.0 Slot?