playswithfire

i was considering buying one from a guy on craigslist, its new in the bow with the plastic wrap on it, now heres my question, when i open it can i activate it on my computer lik the old iphone or do i need 2 go 2 a cingular store, also right now i have a blackjack with unlimited text and 400 minutes, can i just put then sim card in the iphone and have it work or do i need a certain plan, if so can i cancel my blackjack service,(im still in the 2 year contract so would it cost money, if so how much?)