What PSU you require will mostly depend on the graphics card you opt for. Do you know which one you'll be getting?
Don't worry too much about single/multiple rails - instead focus on getting a quality branded one with enough on the rails to run your system well.
Modular ones are more expensive of course.
Some of the PSU brand names I would trust: Antec, Enermax, Corsair, Tagan, Be Quiet, Seasonic, Silverstone, Etasis, OCZ / PC Power & Cooling, Zalman, Xigmatek, Hiper, FSP, Akasa, Thermaltake, Coolermaster, Gigabyte, Silver Power, etc (may have missed a few). G