Try running this: Thermaltake Power Supply Calculator
I'm not necessarily advocating Thermaltake, but it will give you an idea as to what size you need.
As for single-rail vs multiple-rail, opinions vary. If you have the right kind of system and are good at figuring out load balance calculations a multiple-rail is OK in my opinion. I prefer having single-rail because I think it takes some of the guesswork out of it.
XFX has a short video touting their "EasyRail" system here: Power Supply Series - Pro-Series