As for the FW cable, you don't need to go buy something if you don't have one already - either network the computers wirelessly or hardwire them to the network (recommended because it's faster) or connect them directly to one another with an ethernet cable.
I migrated (for a friend) from one iMac to a newer one and was unsuccessful with ethernet. After an interminable amount of time, Migration Assistant in the newer iMac was unable to find the old one, even though I followed all the prompts and entered the correct numbers. I had the foresight to be prepared for this and had brought the Moshi adapter and a FW 400 - 400 cable with me. Rather than try to troubleshoot, I reconnected the two machines via Firewire and the migration was finished 40 minutes later.