It's not safe to run more than 500mA per USB port and for a total of 1.5 to 3Amp depending on your power supply. The computer that i'm using right now has a busted USB power because i tried a Samsung spinpoint mobile 2.5' rated at 0.8amp. When i pluged it into the USB ports, a capacitor in my computer blew up resulting in loss for power for all the USB ports.
There's a fix for this if this happened to you, get an old cell phone charger rated at 5v 0.5-1 amp, cut the charger's phone connectors, cut a USB cable and leave the male side, connect the 2 cables together, black wires connects to black wire and red wires connect to red wires and green and yellow or any other colour is useless. It seems to me that the power for all the USB ports are shared, thus if you plug in the modified cellphone charger in a USB port in a computer, it acts as a power boost for those power hungry devices.