OEM can only be used once PER MOTHERBOARD. You will be fine. Samsung SSD's come with migration software that allows you to transfer everything without having to re-install windows.
I have a 256gb 840 pro and its awesome.
As stated above, you will be fine. There is also plenty of free software that can do the job if you don't get a samsung SSD. Also I would like to state that I hate microsoft's license policy. If they didn't basicly have a monopoly on the market no one would have this single install per motherboard BS.