If I build a new system with a 64 bit processor, do I need to make sure that I have a 64 bit OS? I was thinking of throwing one together sometime soon, but wanted to use my old XP on it until I feel comfortable with the Vista patches, drivers, etc.
No, 64Bit processors will run 32bit XP without issue. In fact, 64Bit XP isn't that great - friends have complained about lag issues and lack of drivers for it.