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.