The issue is more than one thing, it's all connected. To start, games like GTA V work best with a quad core. You have a dual core. So, you can stay with the CPU you have if you understand and accept that max settings in games like that are not possible no matter what card you have. Normal settings, maybe even a couple set to high, are more likely with this CPU. I'd suggest a 1050, it's a good mix of low cost and decent performance for something like GTA V. A 1050 Ti is obviously a better card but will your system really take advantage of that power? I don't think so, not enough to justify the cost. It's not worth it at 720p.
The alternative is to buy or build a new computer. There is no worthwhile CPU upgrade for the system you have.