I have an Athlon 64 bit processor and am running XP Home which is a 32 bit OS. XP also comes in a 64 bit version.

There are two DVD's packaged with the "Ultimate" edition of Vista, one is a 32 bit and the other, you guessed it, is 64. The product I have is an upgrade version which requires an existing OS to be in place.

You can use the 64 bit version of Vista only if your upgrading an existing 64bit OS like XP x64. If you have XP home (32 bit) and a 64 bit processor like me you'll have to purchase and install the 'stand alone' version for computers without an OS. MS calls this a "full package product" and it's costs about 5O% more.

That's what little I know so far.