I assembled a PC from scratch about 3 year ago. It has worked well for all these years other than a slight heat problem that I solved by adding an extra fan. I used a ASUS P4P800 SSE mobo P4 CPU, nvida FX5200, 2GB ram. All of a suden it went dead other than the fans, a green light on the bottom of the mobo and the blue power switch light. The poer switch does not turn it of, just the cut off in the back. My wife uses the rig and she rarely turned it off, it did freeze occasionaly. I tried disconnecting most of the ram, hard drive, video card and then powering up nothing. I was thinking it was the PSU because she had it running for "3 years straight" ;)then I was think the CPU may have finally expired form the old heat issue.
Hate to buy both or maybe a new mobo. any suggestions.

It could be one of many things but I would try a new power supply first, it is the cheapest and can be used if you decide later to upgrade. Even though the fans and lights are working, the PSU could still be faulty. You have to decide if you will pay a tech to find the problem or start buying the parts it could be faulty and hope you find the right part early. You have tried most of what would be suggested here. Can you try a power supply from a friends PC?
PS. Many machines are run for well over three years straight with no problem, others burn out before 1 day of running.

Do I understand that the PSU fan is turning? Sometimes certain voltages will fail without others. Most PSU fans run at 12v. You could plug in a usb device (one with lights) to check the 5v.

Another idea is to clean out the dust. Be sure to avoid electro-static discharge.

To check the RAM, you have already removed some of the RAM, now pull the other stick(s) and some that you pulled the first time.

Good Luck