Did some research, the 6pin can carry 75W, and the 8pin can carry 150W.
However, gauge 18 wire can handle way higher wattage than the specification of 150W.
So if one were to have a 5970 300W TDP card, it is theoretically possible to pull all 300 watt from the 6+8pin cables only.
I missed this the first time around. While the PHYSICAL connector can support >300W just with a single PCIe connector the SPEC doesn't.
A little bit of history on how AMD/NVidia totally fucked everything up.
So first video cards were powered only by the bus. Obviously not enough power so in the early days prior to ATX V2.1 (June 2005) both companies used Molex connectors or other propreitary connectors to supplement the bus.
They (AMD/Intel/NVidia/PSU suppliers) agreed to update the standard. They came up w/ PCIe 6 pin connector. Physically it can provide about 350W safely. However AMD/NVidia said. Wow. No way we need 350W+75W for a card. Lets make it 75W. So the spec was ratified and power supply suppliers built PSU based on that assumption. There are PSU designed under the assumption that no device will EVER pull more than 75W from a PCIE 6 pin connector. Once the first PSU was built based on that inadequate spec we were forever fucked because of backwards compatibility.
Well it took about 9 months and AMD/NVidia needed more power. The 8 pin connector has the same number of 12V wires as 6 pin connector. This tends to confuse people. The spec has an "interrogate" ground check in it now. A card attempts to check for ground (ground-sense) on pin #8. If it finds a ground it assumes it can use 150W. If it doesn't then it must accept it can only pull 75W from that connector. Both connector are capable of 300W+ so the 75W or 150W limit is just based on the spec.
AMD will never release a card which pulls 300W from a connector because if they did and it was hooked up so an older "compliant PSU" it would be a lawsuit waiting to happen.
Sadly the spec could have been a lot better had AMD/NVidia been forward looking.