"It depends on the type of hard drive used and the power supply your computer has for the FireWire ports. In theory, Firewire 400-6 pin can deliver 45 watts of peripheral power, which should be more than enough. Try to find how much power your PC delivers on the FW ports.
Biggest problem however is to find a Firewire 400 SataII HD case, without external power supply. On the other hand, if you find a Firewire 800 box, you could always connect it to your PC with a FW400-FW800 cable. 2,5 inch HDD cases with FW are always made for bus-powered operation, so if you can find a 1 TB 2,5 disk, go with that."