In addition to what jsc wrote, some psu's also have limiters on the 12v rails which effectively limit the amps to the label rating. Higher end psu's (Corsair, PCP&C, etc) do not have limiters on multiple rails and can provide the total amps on one rail as noted on the second line as mentioned by jsc.
Generally speaking, one of the components that you should not cheap out on is the psu. I'll never understand how someone can spend $800 on a mobo, proc, and memory and then buy a $30 psu. I highly recommend the Corsair 750TX or 850TX psu's; quiet, realiable, and stable with plenty of amps on the 12v rail(s) for whatever you throw into your machine.