Your system is very similar to my old one (AMD64 X2 5600+, M2N32-SLI Deluxe, 6Gb Crucial Ballistix DDR2, Asus 8800GTS, Seagate HDs, WinXP x64 Pro); and I had problems with it simply crashing on me randomly. Some days it just would not run, other days it would run fine for 24-48 hours before crashing. Never did figure out what was wrong. Tried pretty much everything you did, plus everything else I could think of, including swapping out the RAM, removing/cleaning/reseating the CPU cooler, updating the BIOS, updating drivers, defragging and cleaning the registry, swapping out the graphics card; but no joy. I didn't get the post beep that you did, my post was fine; although I did occasionally get a stop on memory errors. Memtest didn't find anything unusual. I also noticed it would run longer if I let it cool down for at least 8 hours before restarting it.
After consulting with some of the other tech folks at work, best guess was that it was an overheating Northbridge chip. Not sure why it suddenly decided to start overheating, after well over 2 years of perfectly stable running.
Never did figure out for sure what was wrong; since I finally just gave up and upgraded the thing to my sig rig.
Edited by luchog - 2/1/11 at 9:57pm