I already purchased this game in 2008...so I need to rebuy it for Mac??
Well, since Aspyr didn't spend months of expensive development and testing time on the one you bought in 2008 but they did for the version for Mac they are selling now I would say you might expect the answer to be yes. Unless you happened to buy it on Steam and it winds up being a Steamplay release. You might luck out then. I am assuming by the way you worded your question that you bought a Windows version of the game in 2008.