On-board controllers aren't that great. They do the job, but a solid MEH.
Without having the 8 drives to do a proper RAID 0+1 you won't see that speed, especially not with an onboard controller.
Okay, I have just confirmed with my systems guy at work (who just attended a storage conference). The basic industry feel is that SSDs are hype. Transfer speeds are only attainable from a virgin state. After a few weeks time they degrade and aren't as fast given the price/GB ratio of conventional disk drives.
You cannot do what you want to do with a pure RAID solution in a consumer grade product. It isn't possible. You may however be able to have a scheduled job copy the contents of the array over to your backup drive. However I question the necessity of this. Use the other drive for a data backup and keep your OS and data separate.
If it was me? Two RAID 0 arrays, One for the OS and one for data. Have a nightly backup job copy the data to an internally mounted data drive (5 internal drives) and then have a twice a week process copy that to an external drive.