Unfortunately, connecting your iMac by Firewire directly to a router which then connects to the cable modem will Not increase the speed of your Interent connection relative to directly connecting your iMac to the cable modem. Your iMac's network speed is at your ISP's delivered maximum directly through the cable modem. All the router would add is the ability to share the cable modem's output signal among multiple connected computers. I believe you are experiencing the fact that the internal hardware (CPU, RAM, etc.) of your older iMac may be slower in processing the ISP's internet signal than your more powerful MacBook.
Hope this helps:)