The OEM version of Win 7 can only be used on one particular PC. Its tied to its motherboard. If you want to use it elsewhere you'll need to call Microsoft and plead your case. That or tell them the old mobo died and you are buying a new one and need to reuse the licence. They'll usually give you a new key on the spot.
The difference between the B75M and H77M is that the H77 has a better audio codec (marginally, but still), an extra 2 USB 2 ports and an extra SATA_3 port. I'd say its worth it although if you've got the B75 then all that is irrelevant.
Finally, that case has a rather poor PSU, I wouldn't rely on it to efficiently power any parts you buy.