Microsoft has come through in a big way with their Windows Server 2003 Enterprise x64 Edition software which provides high performance for both 32 bit  and 64 bit applications on the same system.  Based on 64 bit extensions to the industry standard x86 instruction set, it allows today's 32 bit applications to run natively on x64 processors.  This software allows for simultaneous new 64 bit applications to be executed in 64 bit mode, which processes more data per clock cycle, provides for greater access to memory and speeds numeric calculations.  What you'll get is a platform that will leverage the existing wealth of 32 bit applications while also providing a smooth migration path to 64 bit computing.  The Enterprise x64 Edition software is best suited for business critical appilcations such as large databases and line of business appications, as well as for Terminal Server deployments that use Terminal Server Session Directory.  It will support up to 1 TB of RAM on 1 to 8 way servers.