Looking for intel MB for Gateway 840GM

I have a Gateway 840GM that won't turn on (as in Nothing whatsoever happens when it is turned on); I tested power supply with Coolmax tester model: PS-224, and power supply tests good. When the system is plugged in, there is a green led on the MB illuminated, but otherwise the system is dead. My guess is that the problem is either the motherboard, the CPU (Pentium D, 3.0ghz) or both.

For starters, I would like to replace the motherboard, but with Gateways proprietary configuration, I can't find a MB that fits. Any ideas where I could find a replacement motherboard? The one in the machine is an Intel D945GBI, mfg June '05. Thanks for any assistance.