some people upgrade hardware so as to be more productive. If you work in complex CAD programs, demonstrate software, etc. you can spend your life waiting for computers to do their job so that you can do yours. Having the latest and greatest fastest machine can mean the difference between selling someone on your new concept software or not.
But, as you rightly say, for most people newer hardware doesn't do much.
I am a real Vista fan too. Vista is awesome on powerful pc's laptops but it really does struggle on some older ones with less memory. If you plan on running Vista on your Mac make sure you get a lot of memory. If you plan on playing newer games on it get something better than the Macbook.