I have the 32 bit XP and 64 bit XP on my machine(dual boot). I haven't had any problems yet. Silo works nice on XP 64, Wacom has drivers for XP 64. I keep my 32 bit XP just in case some new programs I try refuse to work in XP 64 though.
You can upgrade, but it requires you to reformat your hard drive. Also make sure you have a 64bit capable cpu. Changing from 64 vista to 64 7 wouldn't require a reformat, but since you are changing from 32 to 64 it is required.
@Sebi3110: yes all possible the thing though that keeps me off this is that Adobe won't develop anytime soon (not within the next 5 years or so) a 64 bit version of the Flash Player. And no 64 bit support means death to me as I do not want to work with a 32bit Max version. Having 64 bit support is important because whether…