A common scenario.
1.) Check the seating of heatsink, ensure stability and it is on the CPU. (You have done this, no doubt.)
2.) Ensure power supply generates enough power, and that it's functioning. (You put in a solid 400Watt, so I think this is good too.)
3.) Check cable connections for connectivity and for anything burnt. (It's what Indian tech support tells us to do.)
4.) Ensure nothing is shorting the board on the backside and such. (Screws could fall down there and no one would know.)

Considering your situation, I'd say four years is a long time of operation, but that instigates you maintained the computer well, and that its parts were stable in general. Motherboards are troublesome, as it is impossible to tell damage besides a lack of activity. Sometimes we see apparent bulging capacitors as you mentioned, but most of these semiconductors go down silently. Power is often the issue, and if the motherboard receives an energy spike, the current is sure to do some sort of damage.

Thanks Beast, perhaps if the rig had been fitted with an Antec PSU, the spike may not hve done any damage?
Cheers: JCH

Antec is highly respected and easily the most prominent maker in PSUs, but electrical spikes can be overwhelmingly powerful, depending on your electrical situation near the comp.
Next time round, just make sure no more than four major appliances--TV's, monitors, computers, speaker systems, etc.--are fitted to a power strip, and at that, a solid one (i.e. Belkin). It's the current that kills anything, according to Dad. Keep electrical traffic low, and expect better results. Best option, in my practices, is taking up a wall outlet with your monitor and computer, and bypassing the need for a power strip.

Good luck on finding a new board, JCH. Might I recommend MSI or ASUS?

Well put Beast, but unfortunately the stockists here in Australia have no Socket A boards available. The only way to try your luck and obtain one, is at a swap meet, or perhaps on E-Bay.
I was tracking an Asus board on E-Bay, but the bidding has elevated the price to that of a new board already, and this one was used!
I will consign the Athlon 1800 XP CPU to a spares cupboard, until such time as I stumble across a useable Socket A motherboard. I build my own desktops, but am somewhat stunned by the rapidity with which obsolescence strikes in the IT hardware industry. Such is life, though.

http://cgi.ebay.com/Asus-A7V8X-X-AMD...QQcmdZViewItem

I like the look of that one (dunno what RAM you're using, but my friend has a similar model). Too bad NewEgg doesn't to deliver to Australia, they're the best business I've ever seen. A lot of the stuff in my case came from there.

Regarding obsolescence: Yes, my new board (first ECS one got a surge, and I'm so happy its gone) has AGP on it, no need for SLI or that fancy PCI-Express yet, I'm waiting for next year. Then we'll have PCI-Ultra or something ridiculous with $900 video cards with video ports that double as thermometers. Keep lookin' for a board, there has to be something on eBay.