One thing to consider is that XP-64 was a beta. All work was ceased on it, with the concentration on Vista-64. I doubt that anyone will be doing development, based on XP-64. How differently Vista-64 optimized apps will work in XP-64 will be a bit of a "crap-shoot," and will depend on how different the implementation of the two OS's are, to each other. Might work - might not. XP-64 is an evolutionary dead-end, excpt for the elements that MS kept and expanded.