I went ahead and installed 64 bit Windows XP. My system is a HP xw9300 workstation, with dual AMD 64 Opterons, and 8GB of RAM. This went smoothly as expected, because the computer was actually designed with 64/XP in mind. All the hardware drivers were available from the HP site.
I ran tests on all the radio software I currently use, or might use in the future, and all went well.
32 bit applications are supported within 64/XP though WOW64. So far, so good. What doesn't work is any older 16 bit Windows applications, and some 32 bit applications that have 16 bit installers.
Although the computer seem to run great, I didn't see any noticable improvement in any applications. That is because none of the applications I run are written to take advantage of a 64 bit OS. I had a dual-boot 32/64 XP configuration, so it was easy to compare. Bottom line, I went back to running WinXP Pro 32 bit. A good case for, "If it works, don't fix it!"
Thanks to those who responded with your comments.