It depends on the setup, though. If you have 5 hard drives, and no CD-ROMs, a low power video card, and nothing in PCI slots, you'd still need a 300w+ PSU, because you're using a lot of a certain voltage.
Apple? Macintosh? What are these strange words you speak?
Yes, I usually recommend double the needed size simply because the rated sized is peak, not continuous, and with the numbers fudged. I just was letting him know that even if he got one four times the size he needed, it would only output as much power as the system used, so it wouldn't affect his electric bill.
HERES where it gets complicated. I use a commecial grade 250 watt power supply. It has higher nominal ratings than most 300 watt ordinary power supplies. So I usually reccomend that anyone serious about their power supplies read the fine print-they will tell you what amperage each voltage is rated at. How much you need of each depends on your configuration, but concerning fans, most people do not need to worry about the 12v circuite.
Look for a rating of at least 15A on the 3.3v for typical systems, 18A on heavily equiped Athlons. 25A on the 5.5v is usually adequate. Since these are shared, the combined wattage should be at least 150W. I have never seen a power supply that would not supply enough 12V current if it met those other requirements.