Ideally you would want to split out the individual types DBs onto their own RAID volumes (TempDB one array, LDFs a second array, MDFs a third and any high volume DBs onto their own arrays).
The problem being, if you are using DAS internally you probably will not be able to afford a RAID 10 for each one. My suggestion is that you consider the amount of volume you will be getting. SharePoint is going to have the same sort of IO patterns as you typical OLTP database.
What you really need to do is use tools like SQLIO and SQLIOSIM to stress test the IO subsystem to ensure it is performing to your needs. It is very hard for anyone to tell you what those are as you should know your user base and the project specifications. Here are a few links to get your research started. Do not cofuse SQLIO and SQLIOSIM. They do perform tests in different ways so use both.
Its all about the requirements. I have seen SharePoint put on a single server with local SQL Server that had all the databases on a single volume perform just fine for SMBs with a low number of users. Test, test, test!