According to Microsoft internal documentation, most product editions of Windows Vista (code-named Longhorn), the next major Windows version, will be available in both 32-bit and 64-bit (x64) versions or will support both architectures via a single installation. Microsoft is dividing the Vista product line into seven discrete editions (nine if you include European-based N editions)--Vista Starter, Home Basic, Home Premium, Home N, Professional, Professional N, Small Business, Enterprise, and Ultimate Editions--and will differentiate them with various features. Part of that differentiation will involve the number and type of processors and the amount of RAM the products will support.
As you might expect, Vista Starter will be the most constrained edition. It will support one 32-bit microprocessor and up to 256MB of RAM. The screen resolution will be limited to 1024 x 768 (up from 800 x 600 in Windows XP Starter Edition).
The other editions--Vista Home Basic, Home Premium, Home N, Professional, Professional N, Small Business, Enterprise, and Ultimate Editions--will ship in both 32-bit (x86) and 64-bit (x64) versions (or will support both architectures). All the home-oriented editions will support one processor, whereas the business-oriented editions (and Ultimate Edition) will support two.
Home Basic and Home N will be limited to 8GB of RAM, whereas Home Premium will support up to 16GB of RAM on both 32-bit and 64-bit PCs. All the other products will support the maximum physical memory size on 32-bit systems and up to 128GB of RAM on x64 systems (although Microsoft could increase that amount in the future because that figure is soft-limited). For more information, see the article "Windows Vista Product Editions Preview."