The case and power supply unit (PSU) are often sold as a unit, although many vendors do sell them
separately. The power supplies included with lower-cost cases are often of lower quality or provide an
inadequate amount of power, so you might want to replace the existing PSU with one of your choosing.
There are a multitude of chassis designs from which to choose, usually dependent on the motherboard
form factor you want to use, the number of drive bays available, and whether the system is to
be placed on a desktop, on the floor under the desk, on a shelf, or in some other location. There are
cases with extra fans for cooling, front panel I/O and audio ports, removable side panels, and motherboard
trays to make installing a motherboard easier, as well as cases that require no tools for assembly,
rack-mounted versions, and more. For most custom-built systems, a mid-tower case supporting an
ATX or microATX form factor motherboard, along with an ATX12V 2.x or EPS12V form factor PSU, is
the best choice. The ATX12V 2.x and EPS12V PSUs will have the 24-pin main power connector used
on the latest boards, while EPS12V PSUs will also have an 8-pin +12V processor power connector
instead of the standard 4-pin version used on the ATX12V units.