Ryan T. said:
About equal on many tasks, but when using applications that are multiprocessor aware you would see signifcant performance gains.
This is basically true, however even for apps that aren't multiprocessor aware, the dual CPU machine will perform better simply because processor intensive apps can take up one CPU while the OS and other, less intensive apps continue to run on the other CPU. I've noticed that even when I've got a render or encode process maxing out one of my CPUs on my dual 1.8 GHz G5, the OS and other applications remain snappy. Not so on my PowerBook (haven't used a single G5, so can't compare to that).