As you were insistent it was the memory I again tried 4GB of mine in the machine. It POSTED and booted fine. I've just played COD5 on it for almost 45 minutes and restarted it myself 5 times. All without a problem so you're right and it has to be the memory.
I'll RMA it. Leave my 4GB in her machine that we know works and get another 4GB for myself.



I would increase the voltage in the RAM by the smallest possible increment.
And pay close attention to your GPU Temp, use RealTemp for this.

Also, I doubt they bothered to check your RAM, so just because you're getting new RAM it doesn't mean the old ram was tested and proven faulty - so you might plug it in and have the same issue - especially if it's the GPU Temp rather than the RAM.