To set difficulty for training missions, just go to Single Missions and adjust the realism settings there. You can also do the same in the career options, but I'm guessing you probably haven't started one yet.
I run almost the same setup as you (RAM and GFX card are the same) and GWX runs fine for me, although anything particularly extravagant like a tanker exploding slows my frames down, obviously. For the most part it's all smooth.
I personally haven't played War Ace Campaign or NYGM (The other two supermods around here) so I can't say much for them, although they are highly recommended by many, but GWX is definitely a top-notch mod.
Oh, and welcome aboard!
