A simple backup will do fine. Also, go with the "Archive and Install" option. It'll keep all your user files as they are, but archive your current system files and install a fresh OS. Your old OS files will still be accessible in case you want to move some things back to the new OS. If you don't care about the old OS files, you can just trash them.

Archive and install is the cleanest way to go if you'd rather not wipe everything.