Actually, I got both machines working, and they've been humming along just fine for several months now. The trick turned out to be getting the correct video cards installed. I found a guy locally who had a couple of Nvidia cards out of the original machines. About $65 each, and I was off and running.
These things really hum. Even the "slow" one is pretty spiffy - my son is upstairs playing Doom III on it right now without a glitch.