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You will get Lion for free if it is not already installed on the MacBook, but as Apple seems to hold all further releases (including the MB, MBA and Mac mini and Mac Pro), Lion will be probably already installed on those ones, if you decide to buy an updated MacBook.

http://www.apple.com/macosx/uptodate/

And yeah, one thread is enough, and btw, this gets asked a lot.
 
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