No problem running Exchange 2003 on 64–bit hardware, as long as you are running a 32–bit OS. Of course, Exchange 12 will require 64–bit hardware and a 64–bit OS. No Itanium though, just x64 or EM64T.
This article contains more info: You Had Me At EHLO... : x64 bit hardware will run Exchange 2003 on 32 bit Windows



