Since you've paid the premium for a Velociraptor I assume you want good performance. If that's true then you should consider buying an SSD (Solid State Disk) to hold the OS and applications. You don't need a huge SSD for that, and performance will be much, much better than a mechanical disk.
Eventually I would like to buy SSD, but they are so expensive.
So you suggest when I eventually go SSD to use the Velociraptor as the secondary drive with of course the OS on SSD and not use RAID at all?