ALL the Mac's firewire ports 400/800 do power small portables external devices like external 2.5" hard drives or iSight firewire cameras. 3.5" external firewire drives need to be powered via a AC adapter. USB ports also power small external devices but sometimes they need to be connected in two USB ports to get enough power. That's why the USB cable that comes with the device often has a mini USB connector and two regular USB connectors.
It's surprising your two firewire ports stop working simultaneously. Do you have the firewire 800 to 400 cable adapter to test the iSight in the 800 firewire port of your PB cause I'm pretty sure you don't have the 800 port on your PC machine and your 400 port may be faulty on the PowerBook. Which cables do you have in hand ?