FYI, all editions of the next release of Windows Server are x64/ia64 processor architecture only; there are no more x86 Windows Server builds beyond Server 2008.   You will, however, be able to run your x86 binaries on Windows Server.   32-bit binaries will be dependent upon the "WoW-64" emulation layer (actually just a thin thunking layer on x64, and a full instruction set emulation layer on ia64).   Software Developers will need to ensure their applications are thus compatible with x64 Windows.   It's a good idea to learn more about the potentially complex issues associated with running x86 or mixed x86-x64 based binaries on Windows Server.  

Fortunately, it's a rather mature topic as x64 Windows Server editions have been available for many years now.   Here is an excellent online resource that will suffice to answer most of your questions.  

http://www.microsoft.com/events/series/msdn64bitwin.mspx

Also, be sure to review the resources at the 64-bit Portal here:

http://www.microsoft.com/servers/64bit/overview.mspx