Use the following guidelines to choose a power supply appropriate for your system:
Above all, make sure the power supply you buy fits your case and has the proper connectors for your motherboard. If your motherboard includes the ATX Optional Power Supply Connector, buy a power supply that provides that connector. Consider buying such a power supply even if your current motherboard does not require that connector so that if you upgrade, the connector will be available.
Some sources recommend adding up maximum current draws for all system components and sizing the power supply on that basis. The problem with that method is that it can be nearly impossible to determine those draws for all components, especially motherboards and expansion cards. We recommend using the KISS method instead, as follows:
For a desktop or mini-tower system with a low-end processor, 128 MB or less of RAM, one ATA hard disk, one ATAPI DVD/CD-ROM drive, and zero or one expansion card, install a 300W ATX12V power supply.
For a desktop or mini/mid-tower system with a midrange processor, 256 MB of RAM, one or two ATA hard disks, a DVD/CD-ROM drive, a DVD/CD writer, perhaps a tape drive, and one or two expansion cards, install a 350W power supply.
For a mid- or full-tower system with a fast processor, 512 MB of RAM, one or two fast SCSI or ATA hard disks, a DVD/CD-ROM drive, a DVD/CD writer, a tape drive, and several expansion cards, install a 400W to 450W power supply.
For a full-tower system with fast dual processors, 1 GB or more of RAM, two or three fast SCSI or ATA hard disks, a CD/DVD-ROM drive, a DVD/CD writer, a tape drive, and all expansion slots filled, install a 450W to 600W power supply.
Obviously, individual configurations vary, but generally following these guidelines ensures that the power supply is adequate for the current configuration and has some room for growth if you add components. If in doubt, buy the next size up.
Regardless of your current configuration, take case style into account. It is senseless, for example, to install a 200W power supply in a full-tower case. You might just as well buy a smaller case because that power supply will never support even a fraction of the number of devices the case can hold. Neither does it make sense to install a 600W power supply in a mini-tower case, which simply does not have room for enough drives to require all that power.
Any ATX power supply you buy should be ATX12V-compliant. Even if you're not using a Pentium 4 in that system, buying an ATX12V power supply protects your upgrade path if you decide later to install a newer motherboard. We expect that an increasing number of newly introduced motherboards, both for Pentium 4 and other processors, will begin including the 4-pin +12VDC connector. If your power supply doesn't have this connector, you may need to replace the power supply when you upgrade the motherboard. (You can use an ATX12V adapter cable to pull +12VDC from a drive connector to the motherboard, but that doesn't guarantee that your original power supply will work with motherboards that require +5VSB current higher than required by ATX 2.03.) Buying an ATX12V unit now avoids that needless expense later.
Don't assume that the power supply bundled with an inexpensive case or a $20 unit you find on the sale table at the computer store is adequate. It probably isn't. A good power supply costs at least $35 for a basic system, $50 to $75 for a mainstream mini/mid-tower system, and $100 or more for heavily loaded, full-tower systems.
Some big-name computer makers (notably Compaq and Dell) have used proprietary power supplies in their systems, although this practice is fortunately less common nowadays. If a proprietary power supply fails, the only source for a replacement is the manufacturer, who may charge literally $500 for a power supply equivalent to a standard $50 unit. If you need to replace a proprietary power supply in a system that cannot physically accept a standard power supply, it may be cheaper to replace the PC than to buy a new power supply for it. Rather than pay proprietary prices, consider buying a new case and power supply and migrating the components from the failed unit to the new case. You may also have to buy a new motherboard, but you should be able to migrate the processor, memory, and other components.
A similar problem exists with some of the inexpensive systems sold by such retailers as Circuit City and Best Buy. Many of those use inexpensive, low-output SFX power supplies that are prone to fail quickly. For some of these systems, manufacturer policies make replacing a failed power supply quite expensive, even if the system is still under warranty.