The nature of PC gaming means that a developer can't simply target a reference system.
If you're reading this, chances are you probably already have a computer. IF you have a potato, skip to Optimization. Otherwise, read on. To render a game, many parts of your computer work in tandem. The CPU has to process the game and send data over to the GPU to render to your screen. Each of these requires time to render, and when you add them all together, it becomes the total frametime. Normally the time it takes for the CPU to process the game logic is very low, so if you want to squeeze out more frames, it is up to the GPU. However, some games are very CPU dependent, such as ArmA, GTA, and GMOD. Ultimately, to get an idea of what you need, go look at benchmarks. Look to Bottlenecker to find a good reference.
Most games have default settings which will be good enough for most users to tweak on their own. However, if you have a potato or simply interested in squeezing FPS out from your computer, do read on. Many games will have detailed settings pages, and it is easy to just turn everything down in the name of FPS. This is good, but it usually results in a terrible game, appearance wise. Some settings will affect framerate more than others. Do and try to experiment to find the optimal combination. Otherwise, you can hunt down a config online which can optimize the game.