I would suggest you to use raid 1+0 (on a hardware controller) + large SAS hard drives. Search google for a "raid calculator", which could help you do the math. Check out as well for a "RAID Drive Cage", as opposite to tower RAID, which is something, that you may not need at this time. Have you considered cluster or cloud technology as well? Also, are there going to be remote backups? If the answer is YES, then you should consider a NAS solution too.
For large SAN/NAS storage devices, I would recommend you to check "dothill", and for hardware RIDE controllers "3ware".