Ok, first thing is to change out this processor as I'm still waiting for a couple of parts. 2 screws and the entire heatsink apparatus lifts off the motherboard. No problem. I cleaned off the 8400 with some isopropyl alcohol (70%) and dropped it in the socket. Next, I spread on some Arctic Cooling MX-2 thermal compound that I just bought. Nice. Popped the heatsink back in, fired up the machine, and...it works! I wasn't too sure if this processor would work as the machine it was in was treated REALLY roughly. Good stuff. The processor runs at 31°C idle according to CoreTemp. If that is indeed the actual temperature, then I'm definitely satisfied with it.
Processor upgrade successful!