Question : Problem: bad psu cpu or mb?

So here's the course of events so far:
1. Custom desktop computer has been working fine for 5 years.
2. Install a SCSI expansion card.
3. Turn computer on and nothing shows on the monitor.  fyi, no beep codes.
4. Swap out for new video card. same problem, no display and no beep codes.
5. Remove video card.  Receive beep codes.  Bad memory is the code.
6. Remove memory, reinstall video card. No display AND no beep codes.
7. Removed processor to force a beep code.  But still no beep code with no processor, video card, OR memory.
8. Removed motherborad from case, put on towel.  Installed processor, memory and video card. Booted up just fine.  Added PATA devices (HD & 2 CD drives), booted up fine.
9. Reinstalled all the hardware and everything booted up fine.
10. Tried to install the SCSI expansion card again and it booted up w/ the card, but the card wouldn't work. So I removed the card.
11. 2 days later, I try and boot up the computer.  Nothing happens.  No fans, no display, no beep codes, nothing.
12. Test the PSU, passes.
13. Plug PSU back in, nothing.
14. Test a spare PSU, passes.
15. Plug spare PSU, same problem...nothing happens.
16. Remove everything again, place motherboard on towel with cpu, memory,video and psu.  Nothing happens.
17. Then I removed the PSU 4P CPU connecter cable and just had the 20 pin power connector plugged in. Tried to turn computer on, CPU fan started spinning.  But nothing else.  No display, no beep codes.
18. Plugged the 4P cpu power connector in.  Tried to start up, nothing. No CPU fan, no beep codes, no display nothing.
19. Tried spare PSU, with the 4P cpu power connector plugged in and then not plugged in.  Results were the same.

So, I've narrowed it down to the motheboard being the issue.  But what's confusing me is that the motherboard seemed damaged before when I tried to add a card.  And then it came back to life almost on it's own.  Then I try and add the card again because I think it's just a freak accident.  And then all this bad stuff happens again. But now I can't get it to boot up at all.

I'm completely confused on this issue.  I don't know what the problem is because the symptoms keep changing.  :(  My guess is that it's a bad motherboard.  But I don't want to invest much more money into the system unless I know 100% what the problem is.

So what are the experts opinion?  Is it bad cpu, mb, psu, scsi card?  What else could I try and get this up and running?

Thanks

Answer : Problem: bad psu cpu or mb?

Add-in cards pull hard on the +5v rail which is also where RAM is powered from when the system isn't in standby.
The limit (by way of current capacity through the mobo) is 2 amps of +5v per slot.
I think perhaps your SCSI card was bad (fully or partially shorted) and pulled too hard on +5v.
- Causing +5v to drop low, RAM's voltage will also drop too low - No BOOT, RAM error.
Doing it repeatedly may have damaged the +5v distribution on the motherboard.

CPU should be fine.
.