New games at medium settings at 1080p requires more than that CPU and more than a 750 Ti. So in essence you need to build a new computer. Prices are very high right now, so this will be expensive. You can use your 750 Ti to save money, but it will limit game performance.
Alternatively, you can simply buy a used AM3+ motherboard and continue to use what you have. But you'll need to play less demanding games, or at 720p low settings.