A new pc case

[cdgamerz]

Can a dell Optiplex 755 board fit in a BTX case?

 

[krzyimprt]

Can a dell Optiplex 755 board fit in a BTX case?

Extremely unlikely. Usually OEM manufacturers like dell make the motherboard mounting holes specific for that case.
You should probably think about upgrading sometime. LGA775 is older than the hills.

 

[cdgamerz]

What if it's a case for BTX and ATX boards?