A friend of mine has just finished editing his first feature film on an iMac 500. We had to up the RAM to 256 MB, but it runs great, and very stable. too, no dropped frames.
BTW he learnt to edit with FCP 2.0 having finally given up trying to find an editor who was prepared to work on his budget (zero), and he has no prior compter experience, which says something about how well designed FCP is.