I'm getting further. I decided to install SO 10.4 via the firewire cable and it appeared to install. Then I tried to boot from that drive via the firewire cable and I got the prohibitory sign (circle with slash). So I'm not able to boot from the drive on my G4 via my laptop through target disk mode. If I can't boot from it then I can't update it. Any ideas?

If all else fails I can swap the video card, but I'd rather not open it up if I can help it.

EDIT: Ok I was able to update to 10.4.4 through the firewire cable (Apple's downloads for 10.4 updates appears to be down and this was all I could find on another site). I still get a prohibitory sign when I try to boot off that drive either through the laptop using the target disk mode or on the actual Cube itself. Very odd.