Whether buying a tower or desktop (to be discussed in the next sections), it is recommended that the unit conforms to the ATX standard and has at least a 250-watt power supply (300 watts is ideal). Make sure to purchase a case that comes with a tray that allows easy access to the internal components. If frequent upgrades are intended, then make sure that the choice of a case provides enough room for expansion. Look for spare drive bays, ample room to work inside the case, easily removable motherboard mounting plates, and drive racks. Additionally, verify the sturdiness of the case because some of the cheaper ones can be quite flimsy. The table gives a summary of the factors to consider in the selection of a computer case.

A system unit is typically a metal and plastic case that contains the basic parts of the computer system. The three basic system unit styles are desktops, towers, and portables. Each design offers characteristics that adapt the system for different environments. These characteristics include mounting methods for the printed circuit boards, ventilation characteristics, total drive capacity, footprint (the amount of desk space they take up), and whether or not it is portable. The desktop and tower design styles will be examined in the sections that follow. Portables are discussed in Chapter 2 "How Computers Work".