"How do you know it is a ECC or non-ECC?"
It will say ECC or registered. If it doesn't its not. This is a selling factor, companies will put it all over if it is, usually.
"How do you know it is a high or low density?
What is the difference between high and low density RAM?"
If I remember correctly it has to do with the way that it is address in the system. Higher density should be faster, but not much I wouldn't worry about it (<1%). Just by good RAM.
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