Avoid Vista as there are major compatibility problems with a lot of music software and drivers, unlikely older apps will work either.
64bit is the future; however we are in a crossover period at the moment, so no guarantees that everything you want to use will work.
XP Pro is the way to go.
Alternatively wait until 2010 when Windows 7 is released (The Vista replacement) as the reports from those that have tried early Beta versions say it really kicks ass.
Hope this helps

Bill