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.
I'm reposting my comment as an answer because I don't think comments trigger an email notification to those of you who might be subscribed to this topic (they don't for me, anyway)... New graphics card was a bust. It doesn't work in any of the slots on the machine. Still not getting "GPU-Present" LED on any slot. Methinks the logic board is busted. Looks like I can get a replacement logic board on eBay for about $80 - but I don't want to go throwing good money after bad. Can any of you confirm that a faulty logic board would cause the symptoms I've indicated?
I believe some graphics cards don't have native OS X support, so they don't show anything until the OS loads. Some cards have Mac versions, and some have a dipswitch to tell the card whether it's being used on Mac or Windows.