We are in the transition from 32-bit OSes to 64-bit. Windows Server 2008 R2 is the first Microsoft Windows version, that have 64-bit version, but do not have 32-bit one. There are Intel / AMD x64 and Intel Itanium IA-64 versions available.
I heard somewhere that Windows 7 will go the same path, but Microsoft decided to support both 32 and 64-bit versions.
I hope that this is last 32-bit version of Windows, and Windows 8 will be available only in 64-bits.
MSDN and TechNet users are already using these OSes, but other must wait till October, 2009.