The case and power supply are typically sold as a unit, although some vendors do sell them separately. There are a multitude of 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 desktop or floor mounted.

There are cases with extra fans for cooling (recommended), air filters on the air inlets to keep out the dust, removable side panels, or motherboard trays; cases that require no tools for assembly, rack-mounted versions, and more. For most custom-built systems, an ATX mid-tower case and power supply is the recommended choice.

Cases designed for the older Baby-AT motherboard form factor are still available, but the ATX architecture offers considerable improvements. The size and shape of the case, power supply, and even the motherboard are called the form factor. The most popular case form factors are as follows:

• Full-tower

• Mini-tower

• Desktop

• Low-profile (also called Slimline)

Cases that take ATX boards are found mostly in the full-tower, mini-tower, and desktop form factors, although some vendors now use flex-ATX motherboards in their low-profile systems. When deciding which type of case to purchase, you should consider where you will place your computer.

Will it be on a desk? Or is it more feasible to put the system on the floor and just have the monitor, keyboard, and mouse on the desk? The amount of space you have available for the computer can affect other purchasing decisions, such as the length of the monitor, keyboard, and mouse cables.

Of the choices listed, it is recommended that you avoid the low-profile systems. Designed primarily for use in business environments, these Slimline systems have a smaller footprint, meaning that they take up less space on your desk than the typical desktop computer and usually are not designed with expansion in mind.

These cases require a special type of highly integrated motherboard. Older versions used the Low Profile or LPX board. The low-profile version of the ATX form factor is called the NLX. LPX and NLX motherboards often have virtually everything built in, including video, sound, and even network adapters.

Flex-ATX motherboards are used in some of the latest small form factor systems. LPX and NLX motherboards do not have any normal adapter slots. Instead, all the expansion slots are mounted on an adapter board called a riser card, which plugs into a special slot on the motherboard. Standard PCI or ISA adapter cards then plug sideways into the riser card (parallel to the motherboard), making expansion difficult and somewhat limited.

In addition, the small size of the case makes working inside the machine difficult. Some designs force you to remove some components to gain access to others, making system maintenance inconvenient. A flex-ATX motherboard can have one or two slots, although some of these systems have no slots at all.

Most newer cases accept ATX-style boards, which have become the standard for Athlon XP and Pentium 4–based systems. If you are interested in the most flexible type of case and power supply that will support the widest variety of future upgrades, I recommend sticking with the ATX-style cases and power supplies.

Whether you choose a desktop or one of the tower cases is really a matter of personal preference and system location. Most people feel that the tower systems are roomier and easier to work on, and the full-sized tower cases have lots of bays for various storage devices.

Tower cases typically have enough bays to hold floppy drives, multiple hard disk drives, CD-ROM drives, tape drives, and anything else you might want to install. However, some of the desktop cases can have as much room as the towers, particularly the mini- or mid-tower models.

In fact, a tower case is sometimes considered a desktop case turned sideways or vice versa. Some cases are convertible—that is, they can be used in either a desktop or tower orientation. When it comes to the power supply, the most important consideration is how many devices you plan to install in the system and how much power they require.

When you build your own system, you should always keep upgradeability and repairability in mind. A properly designed custom PC should last you far longer than an off-the-shelf model because you can more easily add or replace components.

When choosing a case and power supply, leave yourself some room for expansion, on the assumption that you will eventually want to install an additional hard drive or some other new device that appears on the market that you can't live without.

To be specific, be sure you have at least two empty internal drive bays and choose a higher output power supply than you need for your current equipment, so it won't be over-taxed when additional components are added later.