|"always look for one that has a single 12 volt rail like the Corsair brand for example."|
They may be advertised as single-rail units, but Corsair's very popular TX line of power supplies are actually multi-rail units internally.
I wouldn't be surprised if some of the midrange "single rail" offerings from other manufacturers also share current between two or more rails.
I wouldn't worry too much about the number of +12v rails a power supply has. Good multi-rail units can share current between rails, making them just as reliable as single-rail designs. As long as it's from a reputable company and has a high enough combined amperage rating, it'll do just fine. My previous computer was powered by a Cosair HX with three 18A rails (50A combined). That PSU powered an overclocked 110w Opteron and two overclocked GeForce 8800GTS video cards for years.
DOS/9x Gaming Rig | Athlon 600 @ 672MHz w/ Golden Orb HSF | 384MB | 60GB
Voodoo2 SLI + GeForce 256 DDR | SB Live | Asus K7V | 21" NEC CRT | Win98SE