It's a lenovo m58p mini slim one. I need to a better power supply since right now it is 280 watt. I don't know which one to get but I need a 300 or 400 watt one that's cheap and good quality that fits in my 3 1/2 inch computer case. And if I do get one, do I need to change my motherboard or other components? Or will it not matter
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You won't find anything much higher than 280 watts. I recommend a bigger case that can use a regular atx power supply if gaming is what you want to do. Then you will also have room for a regular video card. Your system wasn't designed for gaming.
And, why did you want to change your PSU? Is it faulty or you want some gaming?