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Can you buy a single core and put a dual core tray in (I thought I heard the MoBo was different some how)?
You can but dual core trays are rarely seen for sale & are wildly overpriced when they are found so if you want to upgrade you are better off selling your single CPU model & buying a dual CPU system.
 
you can buy a dual cpu tray, iv seen them on ebay (they tend to be overpriced for what they are, but if you have the cash and dont care then it's the easy option).
only thing to check is
4.1 mac pro's need 4.1 trays (even a 4.1 with 5.1 firmware needs the 4.1 tray i think)
5.1 mac pro's need 5.1 trays

i think, may be wrong there

and tends to be cheaper to just get a dual cpu macpro
 
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