XP maybe old but it is proven technology, the number of problems and incompatabily issues in vista is amazing, even with microsofts own products (Word etc), I'm not sure what 7 is like but scene as its a beta means that there could be unknown problems people haven't found yet.
really its up to you, if you don't mind vistas problems go for that, if you don't mind maybe losing data or having to rebuild the system, or maybe having some compatability issues go for windows 7 if you need something stable go for XP64bit