I just want to make sure I understand the sequence of events.
You added more RAM.
After that, you state the computer would start, and then shut down.
When you say that, do you mean,
the computer would display POST, and then loading of the game system, then shut down, OR
do you mean that the fans would spin and then the system would shut off.
Did you remember to reconnect all of your cooling fans, especially the CPU cooling fan