I'm not sure if you've already done this, but if you have your camera hooked up through firewire to the Mac and the camera hooked up to the monitor through S-Video, you already have everything you need. Just make sure your camera is set to play through and set your preview (it's under the view menu, but the exact placement differs for different versions of Final Cut) to NTSC Firewire and it should do exactly what you want.