Replying to Windows vista

I've seen a couple threads on this, does anyone who really knows comps know what specs you need to run it? I've got some extra cash on my ID card thing for college and I dont need more textbooks and they wont let me take it off! but I can use it at the univeristy store and I can get any version of vista. I'm also curious what the big difference is between like home and ultimate becuase it goes from $99 to $250... I also heard there's a program that tests your comp and tells you which one will run best for you, anyone know the link???