Gooch

I'm getting ready to build a new system, and haven't yet decided if I'm going with Vista or XP. The one thing I have decided is that I'm going 64 bit. A couple of questions: If I buy the retail DVD of XP that has both the 32 bit and 64 bit versions, and I install both on the same system for dual-booting, will I need to pay for 2 licenses? I have heard some negative things about buying the OEM version as far as future upgrading goes. Will I run into problems with OEM or is it fine as long as it's not a "branded" OEM? Thanks! -Eric Gooch