I have not yet set up to build 5.0 kernels, and was hoping not to get (back) in that business. I *could* put a different video card in the machine, but I don't have any non-ATI spares. I have one from a G3 that's much older that I could try, but that may not prove anything.
I can build a kernel for you easily. That, at least, is one thing that NetBSD makes wonderfully simple:
http://www.klos.com/~john/netbsd_noati.gz
I could also put the original CPU back in the machine to see if it's the graphics or the 7455. I'm reluctant to do that because the CPU swap process is touchy, and a little dust or clumsy move can make for broken HW.
Do the video check first. I agree about moving around CPU cards needlessly.
I could run the machine headless for a while as an easy experiment. It would be easy enough to pull the graphics card, once I'm sure it will boot.........
The little cards which replace the modem card and give you a serial port are not too hard to find. That's what I use in all of my G4 servers. Serial consoles are so much more manageable in datacenters...
John