thanks for all the feedback, very helpful.
Just to clarify a couple of things:
1) when I say security, I mean that a dead drive can be replaced without loss of data and I don't mean as a substitute for a good backup strategy. The idea's often run out though that a backup strategy is a panacea against computer failure but on the failures I've had and not lost data, I've still lost days in setting everything up again and customizing things how I want them and even then you invariably find something that wasn't backed up. Avoiding this altogether with a RAID, assuming it's just hard drive failure, would be a real boon.
2) As I understand it, another benefit of RAID (0) is that striping enables parallel extraction of data from two hard-drives, speeding up extraction of data from computer's slowest core component, hence my reference to improved performance.
With regard to all this, I was interested in riider's suggestion of using an SSD for the OS and choice apps.
I read an interesting article called 10 Surprising Facts About RAID - google it.. saying software RAID is better than hardware because hardware RAID is expensive and if the RAID card fails the RAID volume fails and you need to find an exact replacement card to recover the data, whereas software raid is portable to another machine. Also, SATA 3G & 6G enable hot-swapping of drives and software RAID consumes little processor - claims just 2%-4% on one core in a multicore machine - ie/ irrelevant. He says software RAID works with SSD caching and software RAID supports variable size volumes that you can extend by just adding more disks
I'm interested in thoughts on this.