You can use an old firewire device on a new firewire iMac (backwards compatibility), but I doubt you would be able to use a new firewire device on an old firewire iMac (forwards compatibility).
You can't teach an old dog new tricks (at least not without a hardware upgrade!).
From the Apple website:
Backwards Compatibility
Manufacturers have adopted FireWire for a broad range of devices, such as DV cameras, hard drives, digital still cameras, professional audio, printers, scanners and home entertainment. Adapter cables for the FireWire 800 9-pin connector let you use these FireWire 400 products on the FireWire 800 port.
that's what I thought!