Steam version of Civ.IV runs fine on my two year old MacBook Air. I had to reduce the graphics and occasionally the game play got sluggish, but overall the experience was not painful.
And I have to agree with a previous post that Steam is the way to go if you catch one of their deals. I think I paid all of $7.50 or so for Civilization IV Complete last month.
AND, if you ever get stuck on a Windows box for a long period of time, Civ. IV is one of the many Steam titles that lets you install both the Windows and Mac versions of the game on as many machines as you'd like. (You just can't use more than one installation at the same time.)