The output from dmidecode is nonsense. 5V for DDR2 RAM? 60/70ns? Are we back in the i486 days?
I would not trust that info. Instead, trust what ASUS says. They created the motherboard, so go with their specs:
https://www.asus.com/Motherboards/P5NMX/specifications/
As you can see, the maximum supported RAM is 4GB. You can try for 8GB -- it might work, it might not work at all, and in the worst case it may seem to work but the system will be unstable.
I do have to ask the "why" question. I'm curious. Is it worth it? 8GB for something so dated?
UPDATE: According to reputable documentation, dmidecode output cannot be trusted:
Beware that DMI data have proven to be too unreliable to be blindly trusted. Dmidecode does not scan your hardware, it only reports what the BIOS told it to.