Driver and software support for Vista64 is a bit farther along than with XP64 despite it being newer. As for expense, the cheapest route is to get an OEM copy of either with your build when you order it. Note: this is not the best option if you want to re-use your OS on another build in the future. Also, if you get Vista Ultimate, you will essentially have the ability to install either the 64bit or the 32bit versions out of the box.