The only consideration for going Vista is 64-bit. XP x64 is not as well supported (if at all) compared to Vista 64-bit. Although there may be driver issues and software issues, but things are moving fast and catching up. Most complainers are gamers. People who use it for work or audio are not complaining. Most audio applications are already 64-bit with others (VSTis etc) following very rapidly. Audio drivers are also catching up fast - most already support 64-bit. So for audio purposes, 64 bit is probably the way to go. The capacity to support more RAM is very helpful for audio purposes.
Other than that, XP is still the choice for 32-bit environment.