IMO, it's not what anyone else might think. SQL, Exchange, and Web Services get all the hype, but I think Terminal Services will get the most immediate benefit from the backwards-compatible nature of the x64 architecture. Let's look at some of the benefits that the platform provides over x86 and IA-64:
I don't have numbers, but I expect that you would get FAR more users onto a 4-proc (Opteron) 4GB or 8GB machine running 64bit Server 2003, than the same machine running 32bit Server 2003.
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