LOL, I wish PC Gaming was this simple.
Depending on the games, a good video card, good monitor, good, cpu, good ram, etc...
Seriously, if you're going to get back into gaming, asking questions like this, is not going to help you, you need to spend sometime online reading and researching, it's the only way, and it's going to take some time, because there are a lot of hardware choices out there, some better than others.