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...