What is a good processor speed for FCP 6 real-time playback?
Certainly get the fastest you can afford, but processor speed is only part of the equation.  You should also get as much RAM as you can afford.

It is a fallacy that higher processor speed = shorter task completion times.  A fast processor helps, but RAM is more important than processor speed.  More RAM means the processor can handle more tasks at once.

A MacBook Pro tops out at 8GB currently.  For video editing, I always max out RAM on a laptop.  For a desktop, I might go to 16GB or more if I was planning to work with multiple uncompressed tracks.