It's because what he has in there now is a pair of 128's, equaling 256 MB. If he wants to upgrade to 512 MB, he can't just buy a stick of 256MB because like you've been saying they have to be in pairs.
So to get 512MB with what he already has, he must buy two sticks of 128MB. Otherwise he would have to buy two 256MB sticks and throw away the two sticks of 128MB that he already has.
He could get up to 2GB of RAM if he got rid of the four sticks of 128's and bought four sticks of 512MB.
I think you almost knew that, just didn't put it together right or something.
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