It's a matter of how much power the GPU needs. The socket supplies up to 75w, a 6-pin connector adds another 75w and an 8-pin connector adds 150w. For example the 5670 can use up to @ 63w so it doesn't need an additional connector. The 5770 uses @ 100w so it uses the socket + a 6pin connector. The 5850 uses @ 150w so it gets the socket + 2 - 6 pin connectors. The 5970 uses @ 300w so it gets the socket + a 6 pin and an 8 pin.