If your Mac is a "Core Duo" and not "Core 2 Duo" both of witch are "dual core" then it can NOT tack 4GB RAM.
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If your Mac is a "Core Duo" and not "Core 2 Duo" both of witch are "dual core" then it can NOT tack 4GB RAM. 2GB RAM is the maximum RAM a "Core Duo" MAC can tack.