There's no question that you would want identical drives if you were to set up an array, but even then, on a desktop, there's very little reason to do so. You would only be going from 500 GB to a 750 GB drive. That isn't much of a difference for your system volume. 500 GB is MORE than adequate for that. I'm using a 128 GB drive right now as my system volume, and that's plenty. You have a very large storage drive in the system, so you shouldn't be wasting any time or effort even thinking about needing more system volume space.
As for the BIOS, there should be a manual or explanation from Dell, and it should have come with your system. You can always go in and look around in the BIOS, without making any changes.
If it was my computer, I'd sell the 256 GB or use it in another computer, keep the 500 GB as C, and the HDD as your storage. That's much more than most of us have, in terms of storage space.