[After the fact information]

I have to agree with Und3ertak3r's advice.
Remove the cards.

With this kind of problem, I remove cards one by one with a reboot between each removal, until I'm left with no more cards, if onboard video.

Shorted cards can put a load on the powersupply that effectively shuts down the powersupply, so you swap the powersupply only to find it is not the problem.

Also the powersupply could be compromised and cannot take even a minimal load, so really, in my opinion changing the power supply is usually a good idea. (course you put a BETTER one in)

I would have started removing cards by first taking out the modem. After so many years of finding the modem blown, this card is suspect almost immediately.

Again in agreement with Und3ertak3r, a lot of store bought surge protectors are well .....(you insert the words...).

Let me turn you all on to a real surge protector:
http://www.brickwall.com/

Hope the new motherboard fixes the issue.