Moving OEM to new board

I was planning to build a new machine with new and old parts and build my brother a new machine with my old Mboard/VISTA and parts I no longer need. I bought WIN7 OEM PRO and new HD so far. Now he says he can't afford an new PC for a couple months. I am not going to buy the rest of my new PC until he is ready but I really don't want to wait to take advantage of my new OS and HD. I know OEM is only supposed to go on one PC. In my view half the parts will be the same PC when I am done upgrading. I have understood windows to treat a new motherboard as a new PC. Will I run into trouble reactivating my OEM WIN 7 when I complete my upgrade (switching CPU MB RAM)? Do I need to refrain from in installing it on my existing PC? I seem to recall a similar situation when I bought Vista. I found that my existing PC was not suited for it so I went back to XP until I switched to a beefier CPU/MB. But, I am thinking I never used VISTA long enough to need to activate it on the old system.