Having run RAID quite a bit myself one must remember having all your drives in one box is always an invitation for trouble since hardware failures on a higher order will likely hit all the drives.
If you want to do online backup get DSL instead of cable internet for the faster upload bandwidth.
Get a backup service with versioning. That way if you or a virus delete something it just doesn't sync the deletion to your backup.
I personally use JungleDisk which uses Amazon S3 storage. You can set the versioning controls and you only pay for the storage/bandwidth you use. My bill averages about $2/mo for several gigs.
The decision doesn't have to be logical; it was unanimous.