With a 3570K you can run any video card you wish. I would pass on the SLI 960's sell your card and buy the best single card you can. Because of the scaling some games you will see good gains some games not so much and the single card will give you better all around performance.
depends what resolution and framerate you want, 1080p 60fps a 390/970 is best bang for buck if you want to run everything on ultra. 960 is still pretty sufficient though. I'd wait for Polaris/Pascal but if you can afford to get a new card then sure.
Most games a GTX 970 will do that some game are poorly optimized and would be a mixture high settings. Your average person can't tell the difference in High and Ultra it's usually just a kid thing for bragging rights.