What is a good processor speed for FCP 6 real-time playback? I currently have a 2.2GHz dual-core macbook pro (2007) and it doesn't seem to work very well with HDV 720p real-time playback. I am looking into buying a new iMac this or next year. What would be a good processor speed for rendering and real-time playback in FCP 6 or 7? They currently have quad-core 2.5, 2.7, and 3.1 GHz.
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.