having your system installation on an SSD should speed up mostly everything you do with on computer, so i'd go for it. of course having individual games on an ssd will help with loading them too, but it's mostly noticeable with huge games that load a lot of assets.
moving system files around never worked for me, and from what i've read it's highly discouraged. however, moving your user data is another thing.
so, the way i'd do it:
1. install a fresh copy of Windows on your SSD. make sure it's the first drive on the drive list, so it installs as drive C. it's also important that the new Windows username is the same as the old one, otherwise things will break.
2. after it's installed, move or copy your old user folder and whatever else you need from your old drive, so all configs and stuff are back.
this method comes with a caveat that all of your software will have to be installed back. but it's a good way to get rid of system bloat, and it's refreshing to have a clean system back once in a while, nevertheless having your settings and personal data preserved.
you'll also have to clean up your old drive to recover space.
the thing with moving my user folder around i got from linux, where it's common to have a single user folder work fine while trying out multiple distros, changing disk drives, or even getting a new PC. with Windows, it may be a bit more complicated, but works fine too if you do it right.
for my screensaver, i'm using Wallpaper Engine with a long playlist of animated pixel art wallpapers. i honestly believe Wallpaper Engine is the most useful and enjoyable piece of software ever.