Well it is always necessary. If you get a under-powered Power Supply or one from a disreputable manufacturer, you will have either not enough power when the power supply heats up or "unclean" power. Both of those will make your computer unstable and it is possible that can even damage your parts int he long run.
Generally look for PSUs with Efficiency over 80% under load at 115-120V and with at least 25A on the 12V rail. Most fo the good quality PSUs today have quite abit more than 25A there.
Companies like PC Power and Cooling, Seasonic, Antec, Enermax/Tagan, OCZ make good quality power supplies.
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