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 Celeron or other low-end processor, 64 MB or less RAM, one IDE hard disk, one IDE DVD/CD-ROM drive, and zero or one expansion card, install a 230W to 250W power supply.
For a desktop or mini/mid-tower system with a Celeron or FC-PGA Pentium III Coppermine processor, 128 MB RAM, one or two IDE hard disks, a DVD/CD-ROM drive, a CD-RW drive, perhaps a tape drive, and one or two expansion cards, install a 300W power supply.
For a mid- or full-tower system with one fast Pentium III, Pentium 4, or Athlon ...