12V: single rail or multi rail?

Hi i'm currently working on building a new computer and i came to where i needed to pick out a power supply. I read up in several places about 12v single rail or multiple rail, and it seems the general consensus is that their isnt really a difference. neither one will give you much of an advantage and as long as theyre well built by good manufacturers, theyll each be able to do the same thing.

but then i talked to my friend, who insisted that with a single rail psu, i wouldn't be able to run 2 graphics card (sli or crossfire) or have multiple hard drives. that i needed a multiple rail psu to split the power for that, i tried correcting him that it didn't make a difference but he was sure it did.

these we're the two single rail psu i was looking at:
750w 12v60a
850w 12v70a

and here's the multi rail psu he reccomended instead:
620w 3 12v18a each

the computer im building initialy would be fine with any of these threes, but i kind of want it to be the last computer i build so it being able to last through stronger video cards (including sli or crossfire) and multiple hard drives is ideal. also i'm just noticing the 850w one isnt core i7 compatable, how is that possible?
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More about single rail multi rail
  1. The 620w is actually a single rail unit, there is no overcurrent protection on any of the 3 virtual rails.
    Any of the 3 power supplies you listed would work fine, if you're looking to SLI or Xfire higher end cards the 850 would be the way to go.
    Do you get email from Newegg?
    They currently have a promotion that takes 20% off the cost of the Corsair 850 making it about $100 after rebate, that's a hard deal to beat.

    Here's some reading regarding the single or multi rail question
  2. thanks that jonnyguru page helped alot! and that sale on new egg is what prompted me to buy a psu now instead of later. i just have two more questions:

    what does a modular design on psu mean? is it simply smaller or is there more taken into effect?

    secondly, the 850w psu isn't listed as "core i7 compatible" is this actually a connection it needs or does that mean it simply wasn't tested with it? would i really never be able to run an i7 with it?
  3. Modular = using only the cables you need
    core i7 compatibility, from what I can tell, means that the psu has an 8 pin cpu connector
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