Civilization 3

Civilization 3 has to be one of the best RTS games ever. The Mac version was published by Aspyr. There is so much to manage in this game whether it be keeping your citizens happy, or trading with other civilizations, keeping peace, winning wars, finding resources, researching technologies, keeping order, managing a treasury, building new cities, and so much more. Civilization 3 is a must have for any computer, old or new.

You can buy this game from Aspyr's website: Aspyr

If that's not your style, there are torrents out there...