The GPU is definitely the bigger problem in your system. The CPU, as I said, isn't doing your performance too many favors, but it should be getting the job done. If you can't run Dark Souls 3 at a smooth framerate (between 30 and 60), it's the fault of your dusty old GPU, not your processor.
Thanks. Is there any point in getting an even better GPU than the ones you suggested (like a 970) or would that mean the rest of the PC would be too old to support it?