1) Windows has no "uninstall." Just re-install it on the SSD. Remember to detach the hdd while you install it, or your boot loader will be installed on the hdd and you will actually be starting your boot on the hdd.
2) Uninstalling or reformatting your OS drive does not free up an OS key. When you do another install, either the process will see that you have changed only your drive and not complain (most likely), or you will call Microsoft and they will give you a key, 24 hours a day, 7 days a week, 52 and a fraction weeks a year. I've done it with no problems.
3) You can make awfully good guesses as to what constituted your OS on the old drive. Once you are sure that everything runs from the SSD correctly, delete obvious things from the hdd like /windows, /program files, c:/pagefile.sys....
4) "games and other files." If you mean game saves, they can be loaded from the other drive. If you mean games that you have installed, you will have to re-install them in the new instance of your OS. For the vast majority of games, you will not be able to run them off the copies on your old drive.
This is because installing a game makes entries in the registry, a datastore that Windows uses to keep track of configurations and permissions, and to generally make our lives miserable. Some installations also add files to the /windows/system32 directory structure.
Good luck, and don't forget to do backups to external media.