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How to buy a motherboard
Determine the goal, or the task you want to accomplish with your pc.
Either it's a multimedia machine, gaming, or office use. This is a very important step, as different machines will have different requirements, e.g. an office computer will require a much less powerful machine than one designed for gaming.
Start picking out the other parts first (the cpu, ram, power supply, etc). Choosing a motherboard is a backward process, especially if it's the first time a person is performing this.