Just generally, look for any folders in "My Documents" that are named after the game company or the game itself. If in doubt, copy/paste the whole contents of My Documents to a backup drive.
Then on your fresh Windows installation (after you setup the drivers etc), reinstall all the games you want to play. Then copy over all the files/folders you had saved from "My Documents".
Just be aware that sometimes the game's configuration file still remembers your last hardware, and this may cause some game to not start up. If in doubt, look out for any ".ini" or "config" files in My Documents, and delete them, this usually force the game to start on default settings. Those aren't the same files as your savegames either.
Yes, it's not 100% necessary maybe to reinstall games. This is the plain english method.
Also, consider NOT reinstalling windows. With the last Windows incarnations it's really not necessary. You are not going to save up a whole lot of space, assuming you already take care of defragmenting the drive, cleaning up old files and so on.
Just do this: before you change your hardware, uninstall all the drivers that you can. Uninstall the graphics card drivers, sound drivers, and so on. Basically, you want to uninstall any hardware related software that you specifically installed yourself. Then shut down the computer, switch the motherboard, and start Windows again. Honestly if you bought decent hardware, it will be detected automatically by Windows without any problems. I know this as I have done this myself. Windows will simply detect a new CPU, new memory and so on.
This will save you hours and honestly you won't save any space, because as soon as you connect your new Windows install to internet it's going to re-download gazillions of .Net updates, service packs, Windows Defender updates, and so on.
PS: If you do keep the Windows install (assuming you don't need to move your games to a new harddrive), there is one gotcha: make sure that you connect the harddrive to your new motherboard in the same solt as it was before. So look for switches on the motherboard and make sure your HDD remains in primary position. If you have a secondary harddrive with other programs installed on it, you also want to make sure they are connected in the same position (master or slave). Otherwise what was your D: or E: drive might end up as F: or G: (examples) and this will effectively break your installed programs. Just make sure you connect your HDD and Optical drives in the same master/slave settings.