Machine checks are a bitch. They are generally caused by memory problems (ECC/Parity) or bus errors. Be happy that the system does not boot up anymore. If you would have sent the system in as it was, when it was semi-working... they would have sat it on a bench and run some crappy burn-in software for a few days to come back and find it still running. They would proceed to send it back to you and charge you for a no-fault-found diagnostics fee.

Running 5 instances of WoW hammers the shit out of your resources. The problem could have been a result of an inefficient cooling system. With all that hardware, you better have a kick-ass cooling system. Over the years, your fans slow down or even stop. Running 5 instances of WoW generates a LOT of heat. The GPU/CPU/Memory/Bridges are working at 100%. It is very possible that your hardware eventually failed from the extreme temps(melted... transistors arc'd...etc). The systems should handle this, but again, over time the cooling system loses its effectiveness.

In any case, something is going to need to be replaced. You can try (for piece of mind..cause it won't fix anything) removing all of your memory and power the system on. It's been a while, so I don't know the specific audible pattern of no memory, but you should hear it. If not, then it's something else.

Good luck!