You say application database server, but not what application, # of users, or expected load. Optimally for a production performance SQL server you would have 2 drives in raid 1 for OS, 2 drives in ...
I was taught/learned to create a C: drive for the OS and a D :, E:, etc. for the data. I remember one reason being "if the OS crashes and you have to reload the OS, your data is safe on the other ...