A GTX480 won't use more than 30A on the 12v unless overclocked nicely. The 42A thing is because you'd need some room for the CPU to run too. A GTX480 and a Phenom II/i7 won't exceed 42A.
On that, each rail is distributed. One rail can fuel the card, while the other fuels the CPU. Basically it can give each card 38A, instead of having to share with the CPU (which will get its own 38A).