Dave:
Charlie Russel from the Windows x64 group wrote an excellent
article to answer your question. The following is a copy of his answer to
the same question.
"You might find this whitepaper of interest. The situation is improved wrt
to
drivers on Vista x64, but most of the concerns are still something you
need
to at least think about to see if they apply to you.
There's a useful whitepaper on the Windows XP 64bit web site that I think
should help you decide:
http://download.microsoft.com/downl..._XP_Professional_x64_Edition_Right_for_Me.doc".