Check this out:

USB Grill

Unbelievable.

I feel sorry for that computer's PSU.

Ian A.

I doubt it will hurt the supply but the traces on those cards may be in trouble. The no-name 480w supplies I play with will give you about 38a on the 5v and the PC would seldom use half of that, even if it was loaded.
This 3.8gz machine was running fine on a 150w inverter the last time the power went out, even with a flat monitor and a modem.

BTW if you really want to steal power from the PC, snake a drive power cable out the back. That comes straight from the PS rails.