Before you begin any type of shopping for a PSU, find the amount of power required which your computer system is currently using. If you plan on upgrading your computer system and will be using the power supply, then it is wise to forecast the wattage for your future computer system. One particular way of determining this, is using an online PSU wattage calculator. The one I reference, eXtreme PSU Calculator, is in my opinion, the best one found online so far, and takes into account many of the latest systems such as dual- and quad-processors, overclocking, SCSI, SATA, and much more. In addition to the eXtreme PSU Calculator, eXtreme Outer Vision's website is also a great place to go for other calculators relating to other aspects of your PC.