Most laptops older than a year or so will only recognise a maximum of 2GB, installed in 2x1GB chips. It's only newer laptops that the ability to recognise 2GB chips, and hence up to 4GB max, has been the norm really.
Regarding Windoze using so much RAM, and especially Vista as it's sooooooo resource hungry, well there is a simple answer. Buy a computer with a well thought out, streamlined Operating System! Such as a Mac
But then you know that already obviously RudeBoy.
Oh, conversely, whilst your Mac Pro (which one you got, do you need that much power for what you use it for?) can recognise up to 32GB, many PC desktops could recognise 2 or 4 times that, if you could actually install it all! Or so I'm told by PC experts at least. Of course, they'd then need Microsoft to create an operating system capable of using more than what most people determine the minimum sensible amount of RAM for most of their duties these days!