Assuming you can run your chosen game acceptably, one of the first hardware upgrades you should look into should actually be a proper gaming mouse. Whether you're just getting into PC gaming or simply starting to take it more seriously, a good mouse will make the biggest change in your ability to interact with the titles you want to play. It doesn't have to be fancy; six or seven buttons and a top-class sensor are all you need. If you want to get fancy, though, there are options like SteelSeries' Rival 600.