Won't Boot - 3 LEDs, no chime, keyboard not responding

I know, sounds like a repeat topic, but I've read (& tried) much of what has been posted. There are some super thoughtful posts, but nothing has helped my situation just yet. Here are the facts:

• LED 1 is on when plugged in
• LED 2 & 3 light up immediately when powered on.
• LED 4 never lights up
• There is NO CHIME
• The 2 fans spin.
• The HD spins.
• The display is black and shows no life.
• The wired keyboard is not responsive - THE CAPS LOCK won't even light up.
• The CD drive won't accept a CD.

THINGS I HAVE TRIED:

• Replaced the HD yesterday with a new one.
• Completely disconnected the display.
• Tried all the keyboard tricks at startup (but the keyboard is not responsive)
• Reseated each of the 2 memory cards, including removing one, then the other, even swapping positions, one at a time and both at the same time.

WHAT I HAVE NOT TRIED:

• Replacing the graphics card
• Replacing the PSU or the GSU or the logic board

I'm definitely at the point where I'm close to calling it quits...don't want to spend too much more trying to get this dinosaur functional for my kids. Any new thoughts are most welcome. I have the machine open and ready for surgery!!!

Thanks much for any insights you might have.

Cheers,

David