I would suggest you find out your percentage of reads to writes before deciding this. The limited hardware you have listed here isn't going to support a very busy database but I have no idea what your dealing with in that area or even your disk speeds. . You could make better use of it if you discover you have 85%reads to writes for instance by going with raid 5. Look what you are suggesting for TempDB and Log. That is going to get you at the most 160-200IOPS with 15K SAS drive. A generic answer of separation may actually hurt you more in such a situation.