|The major users of the +12v rail are the CPU & the video card.|
Modern CPU's (Athlon, P4 & newer) get 100% of their power from the +12v rail. Prior to that, CPU's got all their power from the +5v. So if you have a high wattage CPU, 140W for instance, it will require up to 12A from the +12v.
Video cards are the other main +12v power users these days. The PCI-e x16 express slot can provide up to 75W. A 6-pin PCI-e plug can add up to 75W more, all from the +12v rail, & an 8-pin PCI-e plug can add up to 150W, also all from the +12v rail. Power requirements for video cards can vary widely depending on the make/model. There are a few high end cards that require two PCI-e plugs, so you can see how a gaming card can really put a strain on the +12v.
The following explains the voltage/wattage/amperage breakdowns of all the various plugs: