I yesterday returned my GTX 960 as it was faulty, and have bought an R9 290, which requires two 8 pin PCI-E cables. I have two 6 pin PCI-E cables, which supplied power the 960.
The only difference between the 6-pin and 8-pin connector is that the 8-pin has two extra grounding pins, and that the 6 pin cable supplies a maximum of 75W and the 8 pin cable supplies a maximum of 150W.
The two cables I have are 18 gauge wires (18AWG), and are sleeved. Another limitation is that the official specification says that PCI-E 6-pin cables can only supply 75W, but the 8-pin cables can supply 150W.
From reading various forums, it seems that it's possible to use a 6 pin PCI-E cable on an 8 pin connector, but however that it depends entirely on the quality of the wire used.
I don't want to use a converter cable as I want have my sleeved cables on display in the case's side-window.
Can the 18AWG 6 pin cable I have be used to power the R9 290, requiring an 8 pin connector?