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this has been answered, but you "can", good luck finding a heat sink though. Also, good luck finding the actual CPU for under $500 (what the upgrade costs from Apple). If you want dual CPU's, get dual CPU's, you most likely will not be adding a second cpu later.
 
Do you know what socket is used, cus the xenon uses 2 different sockets?
also can u use 2 different possesors ?

Actually all dual and quad core Xeons use socket J / LGA771. Non Xeon Core 2 Extreme use socket T / LGA775.

Xeon are Server/Workstation class processors.

Core 2 Extreme are Enthusiast/Desktop class processors.
 
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