rails are basically a safety feature. rails are there to prevent overloading wires and rails allow psu manufacturers to use lower quality wires, which carry less amps. the power is divided up into different rails.. which is almost synonomous with wires. as the power is divided up, the wires have to carry less charge and are capped off to prevent over a certain amount of power going through the wires, this is the safety feature, as the wires are capped off, this also allows psu manufactureres to naturally use inferior wires as the wires will be carrying less current. single rail psu's have higher quality wires to carry the extra wires and are generally uncapped, and you'll get similiar performance from both types of psus, except for high power devices (excluding video cards) such as thermoelectric elements, compressors, and the like. video cards really like to draw power over either one rail, or in a multi rail configuration, they draw from multiple rails. one rail is the one that supplies the pci e 6/8 pin wire.. and the card also obtained a good amount of its power from the pci bus(motherboard) , which recieves power from the 20/24 pin atx connector. cards such as the 2900xt use 2 connectors so that they can access a third rail of the psu as sometimes 2 rails comes short in a small psu. however with pci 2, the pci bus can carry more power so hopefully we'll be sticking with 1 6/8 connector. um yeah.. so dont worry about the rails unless if you're running a tec,compressor, etc., then you'd need a single rail config.
edit: it can also be argued that psu's with single rail are better because power because power cannot be rerouted on the rails, as they are capped off.. so if you have a rail that is for the cpu and cpu only( as many psus are configed, to make sure the cpu recieves clean seperate power), lets say the cpu is only using 10 of the 20 amps on the rail, well the other 10 amps cant be rerouted to the say the video card rail as the video card rail is capped so that it can only take a certain amount of power, as its capped off to prevent the wire from being overloaded. so those 10 amps would be technically lost, as they have become rendered useless..
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