Checking ATI's recommendations - They call for a minimum 450W power supply, with 30A on the +12VDC rail. This is very much like the SLI recommendations, and as with SLI, power supplies with dual +12V rails seem to be treated as if they were a single rail. so, two 15A rails is treated the same way as a single 30A rail. Whether it's like this with ATI or not is difficult to say, they aren't as clear with their requirements.
I don't claim to understand power supplies all that well, but I believe the separate rails is just a way of isolating the processor from all the other components using +12V, so the CPU gets the cleanest power possible. I think the Amps drawn can shift between rails depending on load (although I'm basically talking out of my butt based on what I've heard and the way SLI certification works).
Here's the power supply everyone should get -
Yes, that's 1 Kilowatt. Along with kicking the crap out of any of your friend's power supplies, this one has enough power for your flux capacitor so your desk can travel through time.
This has all been a joke (well, the last couple paragraphs anyway, and I'm still checking out various options.