The 64-bit Tipping Point
Intel recently (re-) published a somewhat old document from the 2004's, "The 64-bit Tipping Point". It has been slightly updated and is definitely worth reading if you are still considering to move to a 64-bit application model.
The document compares the Itanium architecture to EM64T the prevalent processor architecture aka. x64. Further benefits of 64-bit vs. 32-bit are highlighted. At the end of the PDF you also find a list of helpful Intel resources on the web.