If you have a decent processor (e.g. a quad core or above), then having a RAID card isn't really going to be a big help. A RAID card just has a dedicated processor on the card to help sort the information between the 2 (or how ever many Hdd's you have) hard drives. They tend to be very unreliable. Soft RAIDs (RAID that is controlled by the motherboard) are more reliable but technically slower, not slow enough that you would ever notice.
Personally if it was me I would just set up a soft RAID through your mobo. Plus finding a RAID card that is reliable, going to support 6 Gbps, and has enough for 4 or more aren't going to be cheap, and if they are, its for a reason.
You have a good board. Use it to its full potential.