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 ...