Dell typically uses custom connectors between motherboards and front panels. So unless you -or somebody else- has mapped the connector in the c521 case, you may not be able to install a different Dell (or non-Dell) motherboard in this case.
You'll also need to purchase a new Windows OS license from Microsoft because your current license is tied to the c521 motherboard. And, you'll have to reinstall Windows on the hard drive because the motherboard hardware will be entirely different.
