"With one 6-pin and one 6+2 pin PCI-E connectors, the SeaSonic S12II 520 Bronze allows you to build up a SLI or CrossFire gaming system." - from newegg. That makes sense then. btw, I'm assuming you don't have any firewire devices and such in that prediction, and just USB stuff like keyboard, mouse, thumb drive (all a max of 500mA@5V, 2.5W).
There aren't many devices that actually consume a full 500mw from the USB. Hard drives (without additional power), cell phones, and flash drives are the notable exceptions here. For the most part, your devices consume about 100-200mw.
As for firewire, I don't figure that in for good reason. It is extremely unpredictable. I have never had a device that was powered off the firewire port. I have had many firewire devices over the years though. Yet when you do find one powered off of it, how am I supposed to know how much power it will be drawing?