I had an old asus p5sd1-fm2. The fan went on high revs when I started it and it wouldn't boot or anything, even after removing all the hardware. I reset the cmos, and still there was nothing. So I decided to get a replacement board. I got the same board (asus p5sd1-fm2) from ebay.
Now without RAM it starts and gives me three beeps, and doesn't reset it's self. But the second i put RAM in it just gets stuck in a reboot loop. No other hardware is connected. I have two sticks of ram, and have tried them both on their own, in all of the slots. No luck....
If installing the same RAM from the first board into the second board, sounds like the RAM is bad.
------------------------------"To better understand why you need a personal computer, let's take a look at the pathetic mess you call your life."
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The odds of two memory sticks failing simultaneously is small. Can you try to see if at least one works in someone else's system? Or can you borrow a known working stick to try with both your mobos?
The other part your "problem" has in common besides memory is your PSU, one of the "Usual Suspects".