The Sandy Bridge versions are faster, even when comparing quad versus hex cores. If you want to overclock, they seem to have a fair amount of headroom for it. And the "K" versions are multiplier-unlocked, meaning that you can increase the multiplier instead of the bus speed, which can result in more stable overclocking. They are much cheaper as well, if I am not mistaken.

As for a RAID array, if you are worried about the lack of fault tolerance, just go with a RAID-5 setup. A dedicated RAID card should have support for RAID-5. You lose some drive space but gain redundancy; if a drive fails the array can continue to operate until you replace the bad drive, and you won't lose any data. The dedicated card adds to your costs, but it can provide a very nice performance boost via less expensive SATA drives.