An SSD is faster than an HDD but cost more.
The main advantage is faster boot times, program loading (so Windows can feel slightly snappier though how much system memory factors into this a bit too.)
Game loading helps too, though it mostly helps games like SKYRIM that have frequent load points (not just initial game, but dungeon, building, map jumping).
So...
You could:
1) Clone or reinstall Windows to a Samsung 850 EVO 256GB or similar (see pcpartpicker), or
2) Use that SSD only for games that benefit (you can MOVE Steam games if you want between drives by backup game, uninstall game, then RESTORE the game but choose the Steam folder on the SSD such as "E:\Steam2")
Other:
3) Having multiple drives means you can make a BACKUP...