Could it support 1TB, or 2TB ?
I've heard FireWire 800 can power big external HDD.
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.
Overall, any 1.8" or 2.5" drive with a firewire external case could be powered off the Firewire bus, it doesn't matter if it's FW 400 or 800. The only caveat is that the manufacturer of the case may have not wanted to do so... (Thus you can still get power bricks...)
To my knowledge any 3.5" drive will need external power do to the power draw of spin-up. The power requirement for 3.5" drives is significantly higher than 2.5" or 1.8".