vakkotaur installed it and brought the machine up. That wasn't the entire problem, though. One of the two CPU boards reported a bad fan. Fortunately, I had a spare on the shelf, and that dropped right in. The system's back up as though there wasn't ever a problem.
I don't know if the fan was the real problem; I'll play with that the next time I'm home. I'm hoping the old power supply is actually good, and if it is, I'll put it back in the machine and run it as redundant power to guard against that kind of failure in the future. In any event, I'll replace the fan and keep the old CPU around as a spare.