PSU is first on that list because this unit is the most important part of your computer whether you are gaming, 3D Modeling, Rendering, or making a server. I brushed on this before but I want to put it here too. If you use a cheap PSU it could undervolt your system from day one, die randomly, or send a spike through your PC blowing up parts if the voltage input drops or goes higher. A cheap unit does not regulate the voltage as good as a quality unit, nor can they handle the actual rated wattage. A cheap unit is rated at their peak wattage, a quality unit is rated at what they can handle for long periods of time and usually have an overhead of 100-200w give or take. This unit sends electricity to every single piece in your computer that you put money into. When you get a power brown or surge it can mean the death or life of your parts as well.