If possible, get multiple raid controllers and create multiple raid arrays.

OS you could go RAID 1.

Database, I would go at least RAID 5 but RAID 10 may be a better fit for you. Number of disks in your RAID setup will help to dictate RAID level and performance at that RAID level.

IF you get enough disks, such that you can create multiple RAID arrays, you should consider: tempdb and system databases being on a dedicated array, log files on a dedicated array, and data files on a separate array. These would all be in addition to your OS array.