Laurent has given me an interesting link to an article about Vista, and specially the x64 version.

Unlike with 32-bit XP versions, many hardware devices will not work on XP x64 because of a dearth of 64-bit drivers (32-bit drivers will not work in a native 64-bit OS). Likewise, many software applications will not install or run because of various issues, including a surprising amount of 16-bit application installers and poorly-designed version detection. […]

All Windows Vista editions, except for Vista Starter, will come with both 32-bit (x86) and 64-bit (x64) versions in the box, on separate DVDs. This includes the Home Basic (and Home Basic N), Home Premium, Business (and Business N), Enterprise, and Ultimate editions of Windows Vista. […]

Vista Home Basic (and Home Basic N) with support up to 8 GB of RAM, compared to 4 GB for all 32-bit versions of Vista. Home Premium, meanwhile, will support 16 GB. And Business (and Business N), Enterprise, and Ultimate will all support 128 GB or more of memory. (The “or more” bit refers to the fact that there are no client PCs available yet for over 128 GB of RAM; when that happens, these Vista versions will support it.) […]

Most dramatically, the Windows Vista x64 versions include a new secret security feature that will virtually eliminate remote system attacks for the first time on the Windows platform. This feature, previously undisclosed, ensures that system files load at random (1 in 256) memory offsets at every system boot, compared to previous Windows versions where system files always loaded to the same offset memory location. Because of this change, most (approximately 99 percent) remote attacks will simply fail on x64-based Vista versions. […]

Those hoping to upgrade should be aware of a few issues, too. 32-bit versions of XP can only be upgraded to 32-bit versions of Windows Vista. And Windows XP Professional x64 Edition can only be upgraded to 64-bit versions of Windows Vista (Business and above). […]