I am pretty sure the processor is part of the logic board! Very much doubt you can change it without replacing the whole logic board and thats not guaranteed to work if apple have changed other specs
Yea, the newer iMacs have socketed processors so you can upgrade them. I don't know which one you could upgrade to and if you'd really notice any difference.
I have had a look at the early 2008 service manual and it shows the logic board as single replaceable item with the processor part of that item. It is not possible to order a processor by it's self for this machine.
Not to say that it isn't socketed as I haven't tried separating them but it's not something you would be able to do under your warranty as far as I am aware.
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