The size of a power supply doesn't change how quick your computer is or will be. You NEED an adequate amount of power to run your system safely and efficiently. You can never have too much power, but can easily have too little. Having too little a power supply will result in your computer crashing a lot with BSOD's and critical errors. You can find out how much power you would need by either looking up what power which component needs or using an online power supply calculator.
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