Civilization IV and Warlords work great for me in OSX, the only thing is, I'm not sure Beyond The Sword is out for OSX yet and patches are way behind the PC version so I still play it in Bootcamp to be able to play Beyond The Sword (which is a pretty nice expansion, especially for the single-player minded player) and have the latest patches.
Civ IV and Warlords work perfect on my iMac 2.0Ghz Core 2 Duo, even when it had just 1GB RAM and the little I played on my brother's macbook is fine.
Beyond the Sword has been out for PC for awhile now, the wait for it to be ported over for Mac seems to be taking forever. So if your gonna pay for it you might just might think of setting up boot camp and buying the windows version. I'm honestly tempted to find some other means that repurchasing the windows version.