Correct, OS on the SSD. Install programs and your favourite / most played games on the SSD.
it is easy to move games between an HDD and SSD (you can do it through Steam for instance) so if you stop playing a game but do not want to uninstall it altogether, you can just switch it over to the HDD and make room on the SSD for a different game.
Looks okay to me.
Drop the sound card. And I would add in 2 extra case fans, as the 275R is not brilliant for airflow and only comes with 1 fan. Assuming you really want that case.
Don't buy 2 of those monitors. No need for your 2nd monitor to be 144hz as well. You are only gaming on 1 at a time! If you want a 2nd 1440p monitor, go with the IIYAMA PROLITE B2791QSU-B1
ok what second nonito you sugest since it doesnt need to be 1440p aswel? also i dont mind which case,, what ever would be best for my build