If you have to worry about what kind of current your rails are providing you are most definitely looking at the wrong power supply. Out of all the components in your computer, the power supply is the one that takes the most beating.
Do not ever skimp out and buy a cheap power supply. With that said, lets get started
There's no regulated way of representing a power supply's capabilities. Most manufacturers sum up the current capacity on each rail multiplied by the voltage of that rail. These specifications should be plastered on the side of the power supply. The marketed wattage of the power supply is the sum of the maximum power on each rail. However this does not mean that the power supply is capable of maxing out all the rails at once and as such it may not perform up to its marketed rating.
PSUs have at least 3 rails. 12v, 5v and 3.3v. Good gaming PSUs will have most of their wattage rated on the 12v rails while poor ones will put unnecessary gobs of it on the 5v rail in a vain attempt to boost the numbers.
There are also a variety of other performance implications. How much does the DC voltage swing with respect to AC swing and line noise? I've seen some absolutely horrific power supplies that are very unstable.
How close to the rail voltage are the actual voltages? Is your 12v rail at 12v or is it at 12.5, 11.5? This should not vary
How does the rail voltage change with respect to load changes that are within spec? This is crucial for gaming computers in which graphics cards can go from 0 to fast in a microsecond. If the powersupply cannot handle the transient that results from a change in load then the constant voltage supplied to the rail won't stay constant.
Unlike most PC components, PSUs are very easy to counterfeit and there are a lot of really poor ones out there. They are the Rolex of the PC world. Don't pay attention to the marketing, look at the specifications and professional reviews here on Toms. Don't skimp out and buy a cheap PSU, you will regret it.