I recently added a "Resources" page to my web site, on which there is a list of many music software programs I have tested in Windows XP Professional 64-bit. I have a list of what works and doesn't work, as well as some 64-bit bugs that I've found.
I searched for a long time to find out what worked and what didn't before upgrading to 64-bit, but I found very little information on the subject, and most people on forums were just as curious as I was. So, after pioneering, I'd just like to share with you what I learned ... I hope you find it helpful!