Good. Then all is not lost. I don’t personally know of any device that would output HD video from your Mac’s firewire port.
What you REALLY need to get is an actual broadcast video card. I picked up an old BlackMagic Decklink HD Extreme card a few months ago for a couple hundred bucks. You could run component HD out from that to your HDTV and it would be look great.
The newer cards from BlackMagic, AJA and Matrox have HDMI out, but they’re more expensive and may not work in your older G5. Hopefully someone who knows more about G5s could chime in here.