Yea and just to add a simple explanation for all the hardware parts,
Motherboard: The body of PC. All hardwares are incorporated into the motherboard. Make sure the motherboard can support the type of CPU, RAM and GPU (those 3 esp) you want as the hardware components.
CPU: General Intelligence (if u put it into an analogy) of the computer. For gaming, Dual cores with higher speeds can be better than Quad cores (for example, go for Dual cores with 3GHz rather than Quad core with 2.1GHz, since Quad cores are more for multitasking on the PC). AMD cores are cheaper in comparison to Intel (although the performance is quite good). Look for Number of Cores, Speed of each cores and Cache memory.
RAM: Short term memory of PC. Essential in Gaming and other PC applications. Look for 2 things, memory and speed. For gaming, 4GB and above RAM with 1066MHz speed and up is enough.
Graphics card: Communication skills of the PC. Good Graphics card is a must for gaming, but otherwise, it doesn't matter too much.
HDD: Long term memory of PC. Look for the RPM (if going for conventional hard drives) and the actual space (eg 512GB etc).
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