I've used XP Pro, Vista Home Premium and Vista Ultimate. All OEM versions. On each one, I've transferred them from one build to another with no problems from Microsoft. The XP has been on 5 different computers and activation over the internet has never failed yet. I've gotten a message about having to reactivate becasue of parts I've switched out, but only once have I ever had to call Microsoft and they just asked if I had uninstalled it from the previous machine. I told them the parts from the old one were on Ebay and they gave me a new activation number without problem. Both versions of Vista I've transferred to new builds and reactivated without any problem.
Microsoft is having enough trouble selling Vista without pissing people off by being picky about moving it from one system to another.
If you ever get a message saying you have to call, just be honest and tell them what you're doing. If you're just rebuilding with some new componenets or if you've disassembled the old system, they won't give you any problems.
If you ever need support, you can find out anything you need by searching on the internet or searching the Microsoft knowledge base.