Every GPU is different. Most GTX 960's can go over 1400. Overclocking with afterburner is quite safe, so I would start with whatever offset gets you to 1300, and then keep going up in small steps. When you start to notice instability, dial it back a click or 2. You can also try to adjust your fan curve to squeeze out a little more performance. Then do the memory. I would start at 3600 for the memory. I happen to own a 960 and it's stable at 1418/4140 for nearly a year, but I got very lucky with the memory as none of my other Maxwell cards get anywhere near that memory clock (a 750ti and 2 980ti's).