Sounds like the motherboard is bad. If it was just shutting off I'd think it was overheating, but the fact that you get no display and no lights sounds like a bigger problem to me. I'm no expert of course.
Maybe, i've no mobo lying around so i think im fucked, i was also suspecting PSU but not really sure, i can give the full story to see if it helps.
Invalid ram?
Doubtful, i should try, wouldn't hurt, but it's still not likely