For the regular user I recommend purchasing Windows Vista Basic.  What do you loose? Remote Desktop, Aero, and advanced Networking capabilities.  Don’t know what those are? No problem that is why you should buy Vista Basic.

When buying Microsoft Windows you have the option to purchase a retail version or an O.E.M (original equipment manufacturer/system builder) version.  What does it mean to buy O.E.M? See here.  Most likely you will purchase the 32bit version unless you have a 64bit capable processor.

On this Microsoft site you can read about the different versions of Windows Vista.

I recommend purchasing the software through Newegg or Amazon.