Question : Problem: Power Supplies / PSUs - Watts, Amps Ratings - General Info

I'm looking for some information on power ratings for power supplies. I understand the "wattage" rating of PSUs, and how to convert between watts/volts/amps (e.g. ohm's law). I also understand that that total wattage alone can be misleading. It's all about how much power can be provided on each rail, and when building a custom PC, it is critical that the PSU is capable of delivering the required amount of power on each rail.

So for each of the following rails:

      +3.3V
      +5V
      +12V

I'm trying to figure out the following information:
      1) What components in the computer draw power from this rail?
      2) How would you go about ballparking the amount of power you need on that rail?
      3) What are typical expected values for this rail for
                   a) A normal office workstation (aka one you'd put a ~300w psu in)
                   b) A mid-range gaming PC (aka one you'd put a ~600w psu in)

So, nine pieces of information in this question.

Answer : Problem: Power Supplies / PSUs - Watts, Amps Ratings - General Info

The 5v rail is pretty much defunct now. 3,3v is not far behind.
The majority of power is from the 12v rails, as newer motherboard use DC to DC conversion chips on the motherboard, to provide the lower voltages needed by these chips.
Ballparking is easier done with a PSU calculator that you can find online. The main idea is to provide enough Amps(the real power) on the 12v rail to satisfy the needs of the components. The sticker on the PSU should give the Amp rating for each voltage. The 12v amps are the most important aspect to ensure you have enough power to run the component in the PC.
Here is a great one to use.
http://web.aanet.com.au/SnooP/psucalc.php

This link can give you an idea as to the history of PSU's and the evolution of them.
http://www.playtool.com/pages/psurailhistory/rails.html