[Nagasama]

Explanation: Qouted right from PC Power And Cooling: Power Supply Myths Exposed!
"With all the hype about multiple 12-volt rails (ads claim that two rails is better than one, five is better than four, etc.), you'd think it was a better design. Unfortunately, it's not!

Here are the facts: A large, single 12-volt rail (without a 240VA limit) can transfer 100% of the 12-volt output from the PSU to the computer, while a multi-rail 12-volt design has distribution losses of up to 30% of the power supply's rating. Those losses occur because power literally gets "trapped" on under-utilized rails. For example, if the 12-volt rail that powers the CPU is rated for 17 amps and the CPU only uses 7A, the remaining 10A is unusable, since it is isolated from the rest of the system.
Since the maximum current from any one 12-volt rail of a multiple-rail PSU is limited to 20 amps (240VA / 12 volts = 20 amps), PCs with high-performance components that draw over 20 amps from the same rail are subject to over-current shutdowns. With power requirements for multiple processors and graphics cards continuing to grow, the multiple-rail design, with its 240VA limit per rail, is basically obsolete."

word up....that's exactly why i bought the 550vx instead of the 520hx...and puh-lenty of amps to run SLI gts 512's