That CPU is still a little bit capable in modern games, so I'd prefer upgrading the GPU first. The CPU is kinda old, and it'd be time to upgrade soon, but it's not yet. So my recommendation would be to go with a new GPU, either the RX 580 8 GB or the GTX 1060 6 GB for 1080p gaming. But you can't go above these GPUs, because then you'd see some bottlenecking. So don't go too high with the GPU, nothing above the RX 580 8 GB. You have some good budget GPU options right now, so you have plenty of choices to pick from, as well as plenty of price points. Pick the one that suits you, and you can upgrade the CPU later, around Christmas or even later. But I wouldn't put it off further than April 2019, because by then games would probably begin to use more cores, so you'll need an upgrade by then. Right now, get a good GPU, and start gaming!
Since you already have the games and play them, I suggest using MSI Afterburner to view your CPU and GPU usage while you play. Use your normal settings and resolution. Your GPU should be at or very near 100%, while your CPU should be well under 100%, hopefully far under.
If so, this would mean you still have CPU power to spare(because you aren't close to 100%). At the same time this means you could use more videocard power(because your 750 Ti is maxed). My guess is you will need the GPU first, especially for RSS6.