I think I know the reason for this problem.
I own a Asus P5A, that puppy uses the same chipset, ALi Aladdin V Chipset. When I look through the manual, the maximum configuration of memory it can handle is a stick of 256MB. Or 3x256 thanks to the 3 dimm slots.
So, 768MB would be the maximum amount of memory the chipset 'should' be able to work with.
Hope this helps
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