The replacement motherboard finally arrived today. I forgot to get some thermal paste for the reseating of the heatsink, but managed to scrounge some from the office.
The good news: moving the CPU, memory, and cards over to the new board and replacing it in the case was fairly quick, and everything worked the first time, for a change. Voltages are looking a little bit better.
The bad news: The hard drive still misbehaves. And for some reason it’s misdetecting the CPU as an Athlon 1800+ instead of the 2400+ it really is, even after a BIOS update, so it’s possibly only running at 75% of full speed right now.
It looks like the bulging capacitor may have been a red herring, and the problem lies either in the power supply or the hard drive itself. Oh well, at least this board is more upgradable than the old one; when it comes time to hand it down to the Linux server/PVR, it’ll either already be good enough to handle HDTV video or could be cheaply upgraded to do so.
Update: I can manually set the FSB speed and multiplier and then it properly shows up as an Athlon XP 2400+…but then the voltages fall even lower than they were on the old board, with the 5V bus showing up as only 4.4V and highlighted in red. Red bad. It may have been falling back to a lower speed just as a form of undervoltage protection. And the drive won’t even spin up after that change. That clinches it — it’s almost certainly the power supply.