At this point, you shouldn't change anything. Your computer is about as powerful as it will ever be. If you need more power and speed, then you will need a new motherboard and CPU. There is nothing on the AMD side of things worth getting except Ryzen. This by necessity means moving to DDR4 system ram.
If you absolutely, positively, also require native PCI-EX 3 then a new motherboard/CPU/DDR4 ram combo is also called for here. All computer systems eventually reach the point where you can't simply add on capabilities anymore, the platform itself can't really take advantage of them. Your system has reached that point.