A gtx 960 will work in a system with ddr4, as it has its own ram onboard and doesnt use system memory (mostly). You need to make sure your motherboard and cpu works with ddr4.
GFX card comptibility is determined by the PCI e x16 slot and not by the RAM.
BUT WHY!!!?
The GFX card don't need no stinking system RAM. It comes to class prepared with its own GDDR5 VRAM.
You need to purchase a 16GB DDR3 kit. You can add RAM and it may or may not work. Mixing old and new RAM kits that are identical brings with it a possibility of crashes and other bad issues.
RAM compatibility is determined by the motherboard.
Please post your complete specs. Your motherboard decides which RAM can be installed.
A gtx 960 will work in a system with ddr4, as it has its own ram onboard and doesnt use system memory (mostly). You need to make sure your motherboard and cpu works with ddr4.