I've always been told with dual processors to keep them as similar as possible. I honestly don't know the reasoning why, though.
Blind Arrow, and Didi, most people don't need dual processors. They only become useful if you do a lot of multi-tasking, or use programs that are designed to be able to use dual processors. There are some video editing programs out there that can use dual processors, and a lot of servers are built with dual processors.
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