Inspiron 570 Power Up Problem - Need Help!

Over the last couple weeks I have been having some issues with my computer and now it is completely dead.  The progression was such that when it would go into standby mode, there was no way to get it out of that mode (ie., holding power button) except pulling the plug and plugging back in.  Now, when I tried turning on my computer nothing would happen at all.  I performed the following troubleshooting:

1) unplugged everything, waited a while and plugged it back it in and still nothing

2) no lights from front power switch and no noises (beeps) and power supply fan does not turn on.  green solid LED is on at the back of the power supply that is near the plug.  

3) disconnected all the power supply connections and and shorted the black and green wires in the 24 pin connector and that turned the fan on.  I also plugged in the hard drive power and the DVD drive power  while the 24 pin connector was disconnected and shorted and I could hear both the hard drive and DVD spinning.

4) tried disconnecting the memory and everything other than the 24 pin connector and still nothing

5) cut the 2 wires going to the front switch to short them together to make sure the switch wasn't the problem and nothing.

Given what I tried above, do you think the power supply is ok and the motherboard is bad?  Is it worth going out and buying a new power supply to try and return it if that doesn't fix it?  If you think it is the the motherboard, any advice on buying one off Amazon or eBay?

Thanks in advance for any advice!