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?