You just need to know what kind of Firewire port you have on your iMac.
Older iMacs will have Firewire 400. You need a 6 pin to 4 pin Firewire Cable to connect your camera.
If you have a newer iMac, you may have a Firewire 800 port. In this case, you need a 9 pin to 4 pin Firewire cable.
Click /About This Mac/More Info.../Firewire (under the hardware list). If it says Firewire Bus...Maximum Speed up to 800 Mb/S, that is Firewire 800. If max is 400Mb/S or so, that is FW400.