|It could I suppose but usually something will still happen even with a bad cpu.|
If nothing happens when you push the switch, are you sure it's connected to the motherboard and on the right pins?
Or could there be a problem with the switch? Disconnect the wires and temporarily short the pins with something like a screwdriver blade and see if it fires up.
You replaced both the motherboard and power supply but did you ever connect the new motherboard to the old power supply? If the initial problem was a bad power supply it could have blown out the second board too.
Disconnect all the drives and remove all unnecessary cards and then see if it boots up. If so, the problem is with one of the removed items.
You're not really green until you're soylent green.