Viper Lair posted a quick guide about the differences and features of Microsoft's latest OS.
The Retail versions of Vista have both 32bit and 64bit versions of Vista in the box. As long as the hardware stays the same, while you will need to reactivate your copy, it won't go against your activation counter. And no, you can't have a dual boot scenario with one 32bit and one 64bit running at once, one or the other at any given time only.Windows Vista Primer
