Im trying to make this a sticky so we can limit the amount of questions asked about the subject.Feel free to add to this information so we can have a close to complete guide
Heres a handy guide that I pulled out of a Maximum PC:
Component Power Requirement Lines Used
Older CPU 20-40 watts +3.3v
Athlon XP,64,or P4 60-90 watts +12v
Motherboard 20-30 watts +3.3v,+5v, and +12v
RAM 20 watts per 256MB +3.3v
PCI Card 5-10 watts +5v
AGP videocard 20-50 watts +5v and/or+12v
(+3.3v from motherboard)
CPU Fan 2-4 watts +12v
5,400 RPM Hard Drive 5-10 watts +5v and +12v
7,200 RPM Hard Drive 5-15 watts +5v and +12v
10,000 RPM Drive 5-20 watts +5v and +12v
Floppy drive 5 watts +5v and +12v
CD/DVD drive 10-20 watts +5v and +12v
Cold Cathode Lights 3 watts +12v
LED Lights <1 watt +12v or +5v[/list]
So figure out how much wattage youre going to eat up on each rail(line used) and how much amperage(current) is available per rail.
Once you know this its simple Ohms Law
Example: if you require 150 watts on the +5v rail, then that +5v rail needs to supply at least 30 amps(which should be indicated on the side of your power supply)if it does not then you need a larger power supply.
Keep in mind that while I say +5v rail or +12v rail all power supplies are not created equal and you should stick to name brands, the power supply is not where you want to skimp out on your system.
________Angelina jolie pic