Sounds like the system board died.
Since you already removed all components, there's no need for further testing.
Ebay is a good source for spare parts, just make sure you get the later revision which has six memory slots.
There's a chance that the other power supply still works fine.
From my personal exprience, the Delta DPS-475CB 1A is a rock solid PSU, I use this model in different machines.
Besides that, it has a built in self test feature.
Here's how to use it:
1. unplug the mains voltage
2. disconnect harddisk drives, system board, dvd, everything
3. remove the power supply from the case and put it on a table (this is not a must, but can make things more comfortable)
4. connect the power cable
There's a green LED inside the box, it is located behind the 92mm fan.
You will hear a click, now the fan should start spinning and the LED should light up.
If you see the green light, this means everything is fine.
You can now unplug the power cable.