i don't like bus powered devices unless it's flash based - just from my experience a lot can go wrong when you mix moving parts with unreliable power source. what's the capacity range you're looking for - looks like 200gb-500gb range? newegg.com is where i normally go first for this type of thing, then amazon.
g-drive doesn't seem to have good reviews on amazon - i'd stay away from it but that's just my gut feeling.
this lacie is 250gb for $94 on amazon - that seems like a good price for what you're looking for.
also, is there a reason you want firewire? usb is more standard, especially if you want to plug it into another computer to transfer files.