Regarding windows 7 and switching motherboards

No you will not run into any problems. Do your install as usual and enter your registration code when requested. If for any reason the code is not accepted all you need to do is call Microsoft and register over the phone. If need be explain that you reinstalled.
The copy of Windows you have was purchased by you and you own it. You can install this copy of windows on as many machines as the license permits as many times as you need to.
I have done this several times myself and have never had any trouble from Microsoft.