This is if your comfortable with electronics.
Disconnect the mobo 24 Pin ATX connector. Create a shorting wire to go from the Green wire to any of the black ones to manually switch the PSU on.
Then fluke all the coloured pins to any of the black ones. Making sure you get voltage on each one being it positive or negitive. Just to make sure your mobo's getting power constanly. i.e no dodgy/loose pins.
You can also connect Fans to the 4 pin molex's if you really want to, but they won't be causing your problem.
You'll most likely be right in the mobo's gone, but also try just putting one stick of RAM into it's slot and then swapping over to the other just to check it's not a RAM problem