I would just upgrade the CPU from the i5 to a top of that socket i7. The i5-660 is a 2 core 4 thread CPU, and since it's on a much older socket, you can get used CPUs VERY reasonably priced. You could pick up an i7-880 for about $130ish, or you can get a Xeon X3470 for about $50ish.
The i7 will have higher clockspeeds, and 4 cores with 8 threads and shouldn't bottleneck your GTX 960. The Xeon is 1/3rd the price and will also provide the 4 cores/8 threads albeit at a slightly lower clock speed.
Another advantage of going this route is that you won't have to change your motherboard out. It should drop right in.