I have 2GB of RAM as well on my MacBook, and the OSX native Civ IV runs fine, but really has trouble if I'm trying to multitask at all (like if I have many programs open in the background). I haven't tried Bootcamp yet, as I'm trying to keep my hard drive Mac OS-only, for the short-term. I would, however, suggest it (as I've mentioned on some other threads), both for speed and for mod compatibility.
Even with the speed, I have an unhealthy addiction to that game. Even though I know I'm not necessarily "gaining anything" from playing that game (no storyline, etc.), I still come back to it. In my humble opinion, it's the very best implementation of Civ yet--a nigh-perfect balance of "realism" (well, more believability) and game mechanics.