You should go with a quality PSU, no matter how many rails it has. Some people claim having 1-2 rails with high Amperes is better but I believe if you get a good brand PSu (like Enermax, PCP&C, Thermalright and many more). THere are calculators where you input your build details and give you a watt estimate. Based on that go out and get one and don't be stingy!! The PSU is the most essential unit in a PC!
Multi rail psu divide their power per rail. You can't get all the amps your psu offers on the same rail. Instead, it's divided between them.
Theoretically, single rail psu are better for you don't have to wonder if your amps demand is higher than what the rail you are using has to offer. However, if you are no planning on lots of heavy parts (2x5970, plus 980OC), it won't make a difference.