1- CPU
The CPU is the core of your computer. The faster the CPU is, the faster the computer will be overall. For serious gaming you can either buy an i7 CPU or the AMD FX 8 Core processor. For casual gamers not looking to push the graphics to the limit, i5 or FX 6 Core is enough too. That depends on your budget.
2- RAM
More RAM is always better but only up to a point. For office work for example, 6 GB of RAM might be enough depending on how many applications you have open at the same time. For gaming to the graphic limit 8 GB is RAM might not be enough, depending on the game, and 16 of RAM is great even for the latest games. Size is not all, speed is also important. DDR3 orr DDR4 are the best choices for great graphics..
3- Hard Disk
Is the hard disk important for gaming? Sure and not only the size but also the speed. When loading a big game, the speed is important because the loading takes less time. The faster the hard disk can send the game to the RAM and to the CPU, the faster the gaming experience will be. I don’t like waiting too much for a game to load. Do you?
4- Graphic Card
The most important hardware for gaming. The faster the graphic card is, the better the games will be without graphic lag. When playing only and you want to win, if the graphic card is not super fast, the game will be slow to show on the screen, the FPS count will be lower and the greta graphics the game could have
Notes:
You might want to consider to have a balanced combination of all these hardware parts. A computer with a fast CPU but a slow graphic card will not run the best games at the highest resolutions. The same is true for a top graphic card with a slower CPU. Having a balanced system configuration is in my opinion the best choice.