Gena- yes, FCP very much benefits from a Dual system, with regards to Real Time effects....and honestly, a Dual system with 2 1ghz chips will get the job done pretty good (I don't see the dual 1.4ghz chips adding that many more Real Time effects than there already are).
Also....give Apple a call, because they have some kind of policy discount where, if you buy something a month before the new model comes out, you get money back or something? Anyhoo, you may want to check that out.
Whats your doc about? I'm currently editing a feature about Islamic Arranged Marriage......its taking me forever to edit though (I'm a director first, editor fifth).