I think with so few spindles carving them up into smaller sets just sets yourself up for one or more of those smaller groups to be a bottleneck. I would lump them all together, and chose RAID 5 or 10 based on a) needed formatted disk size and b) IO testing metrics verifying I could get the read/write performance mix I required.
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service