Obviously I can't comment on how Civ IV for Mac will perform on the MacBook, but I can tell you this much... I picked up a 1.83 MacBook on Saturday, and by Sunday night had XP running under Bootcamp, and Civ IV running on XP. It performs pretty well. I turned everything up to maximum, and found that when I exited my first game it had decided to set things to a more realistic and achievable setting, which in this case was everything on medium.
So you're not gonna get the best of the best for graphics, but on medium, everything runs smoothly and snappily.
I wonder if popping in more than the stock 512 will make any difference in that regard.... hopefully I'll find out soon.
But, at least on that front, there is hope for Civ IV running on the MacBook.