XP x64 works fine for me. I bought it to go with Sonar 5 and have been happy for a while now. However, it's no longer officially supported in Sonar 7. I think XP x64 is going to be one of those Microsoft "bastard child" operating systems - abandoned and unsupported long before it's come of age. It's been good to me, and very stable, but 64-bit computing is all about Vista now.
Also, unless you need more than 4 GB of RAM, there is almost no point in running a 64-bit OS.
If you need something rock-solid right now, I'd go for XP (32-bit). If you don't mind living on the edge, you can give Vista a shot.