Actually, it doesn't matter how much RAM you have available. It's a really, really awful design flaw with Civ4. It only looks for so much free RAM, then it breaks once x amount is being used.
That's what the Civ5 announcement "no more MAFs" really means. They realized that system was ********.
Ugh that's terrible. Despite it's numerous missing features I'm very glad that the Mac version of Civ4 got that part right!