First think : MDF and LDF separate on raid 1 Second think : All in one raid5.
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.