If you're going RAID a SAS controller is basicly a must if you want any kind of decent performance. Do NOT use onboard raid if performance is of a higher priority than redundance.
For people recomending SCSI drives, while it is a good choise. Remember that SCSI is on its way beeing phased out by SAS. Basicly SAS is a Serialized SCSI. Most SAS controller also allows you to use SATA discs on them, sometimes with the help of an interposer card, other times not.
If you choose to go with a SAS controller, do not go for the cheapest ones. I've had fairly good experience with LSI controllers, Adaptecs are kind of nice too but i prefer the LSI ones as the controller logs are fairly easy to interpret.
I would defintely recomending going for some kind of redundance, for example a RAID 5 array of a minimum of 3 discs, maybe with a 4th as a hotspare if you need an extra safety net. However if more performance is needed you could go with a RAID 0 solution. You can do a RAID 0 with more than two drives. However, if one drive fails, you're screwed.
Buy a large external HDD that you do nightly backups on, do not keep the HDD connected to the system. Just plug it in, do the backup, let it be until the next time you backup. Remember, RAID is not considered backup. Its purely a redundancy solution.
These are my 2 cents, do what you like with them
EDIT:
Another way to do it is to purchase a DAS box, fill it with discs and purchase a SAS RAID controlelr with an external interface. That way you do not have to replace /change/m,ake room for internal drives.