Whether or not you can re-use your key depends on your license type. If it is an OEM license either purchased or bought from a re-seller / PC company, it will not be transferable to a new motherboard. If it is the retail box copy you should be able to re-use it.
A fresh install would not keep your applications installed. They might still be there, but they are likely to not work.
I have managed many motherboard replacements, only requiring phone re-activation with Microsoft. You can either pre-uninstall drivers from the old system before shutting down and swapping the hardware. Or, just go for it. Shut down, replace the hardware, and fire it back up. Windows is pretty good at getting it up and running enough to at least boot into Windows. (A preparatory step here would be to get all the new system drivers you will need and putting it on some form of external media, like a flash drive)