Okay, went over to CJ's, swapped PSU, it still dies. Swapped RAM. Still dies. Found out one of the cables was interfering with the front case fan - shouldn't be a big deal, particularly since the entire front of my case is a vent, and even more so because the side panel was off the whole time I was testing it at CJ's. Unhooked unnecessary devices. Still dies. All fans are operating properly, nothing feels hot (CPU, GPU, southbridge, or drives).
So, tomorrow I'll be buying a new motherboard, and if that doesn't resolve the issue, I'll be sorely pissed off. But for now, it's half past two in the morning, so I'm going to bed.