What he said. Corsair is a good brand, along with Antec, Seasonic, and PC Power and Cooling. How powerful it needs to be will depend mostly on the +12V amperage requirements of your current system, which includes your video card. If you have no separate video card, and do not plan to add one, a small PSU is sufficient, for example an Earthwatts 380. Otherwise, let us know what you have or wish to get.