RAID

So anyway... what's the pros/cons of the various RAID configurations possible with 8 of 18 GB HDs? ;) Any reason to use RAID 5+0 over just RAID 5? At first glance, while 5+0 allows loss of two disks, that only appears to be the case if both disks are in a separate set of 4. If there's two disks failing, wouldn't that be a 50:50 chance on 5+0 being ok, or being dead? Is it worth the 18 GB to get that 50:50 chance?