When I built up my current PC I chose “Windows XP x64 Pro” for this very reason (plus I didn’t see the point in restricting my shiny new 64-bit CPU).
Sadly support for XP x64 has been mediocre at best. Some major hardware players (e.g. Netgear) have outright refused to produce drivers for it. Others have produced cut-down software that functions, but misses important features when compared to the 32-bit version.
Even Microsoft, who marketed x64 as meeting the “increasingly sophisticated demands for making home movies, working with digital photographs, using digital media” have not issued x64-compatible versions of their “Color Control Applet” or the “RAW Thumbnail Viewer” - both of which would be very useful add-ons for people working in exactly those fields.
Given this experience I am watching Vista 64-bit with quiet interest. A quick trawl around various hardware websites shows that most drivers offered are labelled as Vista 32-bit only. Is history set to repeat itself?