MrMacman,
Under OS X, every application will feel the benefits of two (or four, etc.) processors. If an application is written using in an efficient, threaded manner, it will benefit greatly from multiple processors. But even if it is a poorly written, unthreaded app, it will benefit from multi processors under OS X as the OS will run that app on one CPU (all to itself) and handle the (notable) background OS housekeeping chores, and other well-written multithreaded apps on the other processor(s). Even an old Classic app will benefit under OS X as the Classic environ can be run on one processor alone.
You always win under OS X.
blakespot