if my 720 Dell mobo dies, very expensive to replace original mobo.  I once installed an Optiplex BTX board in 720 case.  it can be mounted.  two issues: power button and case LED no longer work.  these can be worked out if one is patient.  this is for someone who does not want to mod the 720 BTX case into ATX.  BTX being ancient still has benefit of better air flow and less work to replace dead Dell mobo.