All you ever wanted to know about PSU voltage rails: http://www.tomshardware.com/reviews/power-supply-specifications-atx-reference,3061-2.html
What is the model number of your PSU? On one of the sides of the PSU there is a label that will list the amps available on the 3.3V, 5V, and 12V rails. A lot of times, there will also be a max combined voltage listing for a combination of the rails (i.e. 3.3V and 5V max, 12V1 and 12V2 max, or max combined watts for 3.3V 5V and 12V).
The single largest loads on modern PSUs are the CPU and the GPU; both the CPU and GPU pull from the 12V rail.