I need a quiet and durable machine to be running long hours without shutting down.
Inspiron 580 is cheap but is it suitable? Should I get a XPS instead (Phenom or i5 ?) ?
Definitely get a XPS 8300 with Core i5. If you are thinking of running this computer for a duration of lets say days or weeks, maybe even consider getting a Dell Optiplex or Precision Business Desktop, since they are designed to handle long hours of continued operation.