First think : MDF and LDF separate on raid 1 Second think : All in one raid5.
closed as not a real question by Marian, RolandoMySQLDBA, Paul White♦, Mark Storey-Smith, Derek Downey Nov 26 '12 at 14:45
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Depends on what it is be used for really. For lots of writes I would look at raid 1 or 10 not raid 5 which would be used for lots reads (reporting). Unless you have a relatively new SAN then 5 would be ok due to better caching.
A different Physical disc for data and log files if you can. ALWAYS keep backups on a different disk to the data files this should be RAID 5.
As much Ram as you can get.