A mobo's USB header only gives 500mA. That's enough for a passive HUB but if you plan to use devices like usb-powered Zip250, or a hungry webcam, it may not work, cause these devices needs each and every miliAmp.
A common 4-port externally powered USB2 hub need 5V & 2.2 A (4 x 0.5A + 0.2A for the hub itself). Powering the hub from a molex is the easiest way, because the 5v rail of a molex is produced by PWM, not linear regul, so no watt wasted in heat.
Add a relay to control the 5v line from a 12v fan header, and you'll be sure that all usb devices are off when pc is off.
Phidget has 4-port hub powered from 12v, iirc.