Single verses multiple rails is not a major factor. Two rails simply means that the 12 volt output is divided into two parts. Personally, I prefer a single rail power supply with one output feeding all the cables.
Single vs. multiple rails:http://www.jonnyguru.com/forums/showthread.php?t=3990
If you are on a really, really tight budget, treat it like a 400 watt power supply.
Otherwise, buy something better.