The power supply you choose needs to supply enough DC current to run your
components. Beyond that it needs to supply stable, accurate voltages, i.e. the
12 volt rail needs to supply 12 volts (within normal tolerances of 10% or so)
steadily under any foreseeable load, likewise the 3 and 5v rails at their respective
voltages. Cheap power supplies tend to fall down in these areas. There are
several tech-heavy websites that actually throw a multimeter on the PSU in the
course of a review, seek these out and make sure you select a quality PSU.
In selecting a power supply, check carefully that it has the power feeds you
need, e.g. six-pin PCI power, 20 vs. 24-pin motherboard connectors, etc. If you
are planning on running two video cards in SLI (NVIDIA) or Crossfire (ATI)
mode, make sure your power supply is approved for that. Both companies have
certification programs.
There are several calculators that try to help you select an adequate PSU
for your system, such as:
• http://web.aanet.com.au/SnooP/psucalc.php
• http://www.extreme.outervision.com/psucalculator.jsp
Choose an efficient PSU. Efficient PSU’s run cooler and more quietly and
thus do not create as much noise (important if you plan to sleep (or think) in
the same room with it or use it as a media center PC).
More informations about this theme can be found in Silent PC Review,
OCAU Wiki, among others.