Also, once you download the game and get patched up to the latest version, when you go to upgrade with the expansions, you can just log on to your battle.net account, buy it right there, and log into the game. The new areas / new level cap will be available right away. You don't have to download a whole new game and re-install everything again.
Linty, if you decide to try it, I *think* the base installer is still old for the Mac and is PPC, so if you have an intel based Mac, you'd need to make sure you had Rosetta installed from Apple so you can run the old PPC based app. Once it patches and updates though, it is a universal binary (runs on Intel and PPC natively).