Keeping them in a Case is normally  Better for Cooling, but you need Strong Fans to do so.
(Theres Better Airflow inside a case, then without) For Dust, well, Strong Fans just blow it out (and through).
Use Brushless Deltas for that Purpose. Those are Strong Industrial Fans used in Servers.

Hes Right, For mining you dont need real Bandwith (Thats why you can downclock the Memory of the Cards so much)
X1 has low Badwith, but due you dont need much, you can mine with Extender Cables on x1 too.
But you have to use cables with X1 Male and X16 Female. If not , the Card will not function properly. (Im not sure, correct me if im Wrong)

The D3 1333 is Fine with That Board.

Greetings,
Frog