I currently have a GTX 660 Windforce Edition and my friend has the exact same as me. He has recently upgraded to a GTX 960 and has offered to sell me his 660 so I can put it in SLI. My CPU can only have SLI in 8x/8x as my CPU only has 16 PCI-e lanes. I have been looking at a GTX 970 but I don't know which configuration will be faster and more future-proof.