Another calculator:
http://www.extreme.outervision.com/
Quote: Originally Posted by PCPower & Cooling
Component..................Requirement............ .Line(s) Used
AGP Video Card..............30 – 50W.................+3.3V
Average PCI Card............5 – 10W....................+5V
10/100 NIC.......................4W...................... +3.3V
Floppy Drive.....................5W...................... ..+5V
CD-ROM.......................10 – 25W...............+5V and +12V
DVD-ROM.....................10 – 25W...............+5V and +12V
CD-RW.........................10 – 25W...............+5V and +12V
7200rpm IDE Hard Drive....5 – 20W...............+5V and +12V
Case/CPU Fans..............3W (each).................+12V
Motherboard (w/o CPU or RAM).25 – 40W......+3.3V and +5V
RAM............................8W per 128MB............+3.3V OR +5v*
Pentium III Processor.........38W......................+5V
Pentium 4 Processor..........70W......................+12V
AMD Athlon Processor........70W......................+12V
CarPC link above seems to be closest to actual, but most of the calculators underestimate RAM wattage. Compare to PC Power & Cooling's old estimate of 8w per 128MB and you can imagine this could cause some problems. They are easily the most respected PSU manufacturer, because of their powerful, honestly rated power supplies.
Of course, they want to sell you wattage, so do your own test. Try holding onto your RAM for 5 seconds after using it for awhile and tell me it is only putting out 7-10w...
It may not pay in money to be conservative in system power estimation, but it will certainly pay in time.