Why do we need a dule 12v power supply?

I'm getting ready to build a new system and was unsure about the PS choice. I've heard that you need a two rail or dule 12v PS for dule core processors. Is that true, or can I just use my 550 watt PS that I already have? I haven't seen any info supporting the dule theory, just its the "new Technology". So whats the scoop on dule PS's?

I think I have an adapter for the 20 to 24 pin socket.

To my knowledge, dual rail PSUs were created for safety reasons - the idea was that having all the current flow through just one rail could start electrical fires, etc. That being said, as long as there are enough amps to power your components and you're not overloading your PSU, the number of rails it has shouldn't matter.