Well, it has to be a Mac because FCP only runs on a Mac.
FCP runs fine on all the powerbooks, and reasonably well on the iBooks. But when you throw Magic Bullet into the mix, you are talking processor power needs. Most towers take a while to render Magic Bullet effects.
Get yourself the current 15″ Powerbook model. Good speed and good monitor size. The 17″ is too much IMHO, not worth the additional real estate.