Optiplex 330/360/755/760/780 MT case if you can find one and you want to stay on BTX optiplex 780. You cannot use ATX case.  Front panel pinout of Optiplex 330/360/755/760/780 is reported to be the same (no pin K25).  Safest bet is to buy a 780 MT, a complete system probably better than empty case.  If someone you know has a dead 780 MT, you can possibly salvage the case then. [URL]  
because i need to know what case is good for dell optiplex 780 dt
someone can find me a case for dell optiplex 780 dt that fit a 400w psu