I use Final Cut Pro on a MB with 2GB. I works fine for SD content. I usually do not do large compression jobs on it - I have much better equipment for that. I do not use Motion, but do use LiveType. That too runs well. I know a couple VE students who run FCE on eMacs. I would imagine FCE would do considerably better on any MB.
The downside to using FCP/FCE on a MB is the screen real estate. I find it difficult to work in 13". So, I am mainly just using this combination to do a little short-duration work away from my offices (and much better systems). If the MB is your primary computer, and external monitor would solve the screen issue.