Premiere Pro 1070 or 980ti or titan z, 12c or 16c

generally higher clock speeds is better than more cores at lower speeds. so at first glance the e5-2667's look like the better option. the e5-2670's at raw performance would be somewhat more powerful, but that is only good if premiere is able to use all 16 cores. in some scenario's premiere can top out as early as 6 cpu cores. you could test your current 16 cores with premiere, and if its able to stay over 85% while exporting it would suggest the 16 cores with the e5-2670's might get utilized enough to be a good option. but anything below that 85% suggests its using less than 16 cores and the e5-2667's would be better.

the video card is going to depend on your projects too, 2k vs 4k, and how many gpu fx and plugins are being used. you could check how hard the gtx 680 is having to work by watching the gpu usage (load %) in gpu-z.  if its not being used much at all, then you might not need an upgrade. if you still want to upgrade, then perhaps a gtx 1060 6gb would a good option. if you are doing HD projects and the computer can run 2 video cards, you could also consider adding another gtx 680 or similar like the gtx 770, gtx 960. if you are doing 4k then you will want a video card with more memory, 4-6gb+ and stick with something around the gtx 1070's performance or better. i wouldn't consider any of the titan's, they are usually overpriced vs the gtx's. the gtx 1070 and gtx 980 ti are pretty close in performance, so if you found a good deal on a used gtx 980 ti it might be a good option. however the gtx 1070 would have more memory, use less power, and be quieter.