I'd highly recommend not using a motherboard's onboard RAID to run a RAID5. You'll be disappointed in the performance. Onboard RAID controllers lack the cache and don't have an adequate processor to provide speedy parity calculations. If you want to run RAID5 (and especially boot from it) you'll really want to invest in a PCI-E RAID card that has cache and a good fast parity engine. Nicer cards from Areca and HighPoint will have these. Onboard RAID won't.
If you don't want to shell out for a separate RAID card, then my next best suggestion would be to get (1) more drive, and run a RAID 10. That would give you 2TB of redundant storage, and would be very fast (even using the onboard RAID).