Two nighrs ago i got my new compenents to my computer, a Cooler master 820 Case, ABS BZ800 Watt puwer supply, and a few fans. So i replaced my PSU, which was previously a 500 watt supply with the new one, and moved everything into the new case.

So the computer worked just fine for that night, so i left my computer on overnight, like i usually do. So when i woke up today the computer put itself to the POST screen acking for my password (I enabled askign for the password on boot) I put it in and the computer turned off.. Then i turned it back on and nothing.. Everything starts up just like other people say, fans video cards, drives, HDD etc. So i took the PC apart and put it back t ogether (hoping that would fix the problem) and still, it is no working. it starts up but nothing is displayed on the monitors (ive tried 3 different ones and 3 different cables, a DVI, sand two VGA's)

Replacing RAm didnt work, Resetting CMOS didnt eaither. I build computers on the side and genneraly smart with them.. i can usually fix problemswhen they come up.

And tried using my old PSU

ALL cables are connected

PC Specs:
ASUS M2N32-SLI Deluxe/Wireless Edition
7950 GT in SLI
2gb RAM PC6400
250 GB 7200RPM HDD
AMD X2 4200+
all other specs would not effect this

Sounds Exactly the same as the issue I had also an AMD. Surprised to here the 4200+ would also have the issue. I'm also using it in my back up machine never once had an issue. You reset the jumpers already so it pretty much has to be 1 of 5 things.

Video Card
Power Supply

Sorry can't add much else i've experienced the exact same thing with 2 different builds both AMD in the last 2 weeks.
Just to note the first build started locking up when idle too. Left mine on overnight and exactly the same result you're dealing with.