Well, a 2.0 GHz mac with 4 gigs of ram, will feel faster than a 2.4 GHz with 2 gigs of ram. But as mikes70mustang says, the CPU can't be upgraded in the future.
speed goes in this order for me. Processor first, ram second(cheaper than SSD), then SSD. After you get a SSD your basically as fast as your gonna get. Put a SSD in a PB and youll smoke new MBPs(HDDs that is)