Step IV - Do the math
This is self-explanatory, but here's an example system: Athlon64 3200+, GeForce 6800GT, 3 hard drives, a DVD burner and a DVD-ROM.
From the charts above, we can count 4.2A for the CPU as well as 2.9A for the GeForce. Adding in the fixed values for hard drives and CD/DVD drives gets us the following:
3 Hard Drives = 3 * 2 = 6A
DVD burner = 1A
DVD-ROM = 1A
Lastly, we add 1A for the motherboard and cooling.
That adds up to 16.1A of total +12v current draw, or if we split power for a PSU with dual +12v rails, 5.2A for the CPU+motherboard (+12v1) and 10.9A for the rest (+12v2).