Read this on why you should have Single rail PSU!!!!
Why Single Rail? Well there are benefits to having a single rail power supply. You do not need to fret about which rail your components are drawing from, as you may fear the overloading of a particular rail in a multi-rail environment. For this case, there is no such worry. The prescence of the molex and floppy power connectors on many of the recent motherboards are hgihly likely there for this reason: so as to draw from more than 1 rail if you are using a multi-rail power supply.
link here: http://www.vr-zone.com/index.php?i=3036