As you know, that's not enough to diagnose the machine. http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&taskId=120&prodSeriesId=3823602&prodTypeId=12454&objectID=c01643511 shows this is only a few years old but the failure to start up could be anything.
Why do you suspect the PSU? It wasn't clear.
Some folk forget to share "I installed a powerful video card and may have stressed the PSU".
Bob
My HP a6720y locked up while playing TrackMania. The windows key and Ctrl Alt Del had no effect so after about 2 hours I pressed the start button until it turned off. Now it wont start. The power light comes on, the fans start and then stop after a second or two and then tries to reboot. I get no beep codes, no display. The green light on the PSU is on but blinks when the computer tries to start. The Geek Squad guy said most likely MOBO failure and would be cheaper to replace the computer. Do you think PSU or MOBO? I have unplugged everything in the case to find a bad component and nothing changed. It doesn't start or run at all before shutting down to reboot.

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