Actually a single rail is better than multiple rail, as it allows any device to draw more load. The multi rail psu's only exist due to regulations. To give you an example:
PSU A: 3 12v rails at 18A = 550 W approx
PSU B: 1 12v rail at 36A = 430 W approx
Now you'd think PSU A is better, but if you had a graphic card with 200 watt draw, but rest of the system not taking more than 200 watts, PSU B would be able to support this, but PSU A would fail.
I have the modular Corsar 620 watts one, I can swear by it. Its much better than any CM psu I have used. I suggest read the reviews for it and buy it!