SQL Server - Local Storage Planning

We’re architecting our MOSS production environment and trying to determine how large the disk drives should be for our data servers.  We’ll be connecting to a SAN for all of the databases but I’m trying to determine how much space I need for local file storage on the SQL server itself.  I’d really like to use 15k drives but I’m limited to about 100 GB of storage with the preferred vendor. 

Do you think I would need more than 100GB of local SQL server storage for a single farm MOSS implementation, 17-20 content databases/site collections, ~1 million documents, 400 concurrent users and 10-15k item-level hits per day?  I'm having a hard time even determining what is actually stored on a local SQL server HDD when it's connected to a SAN for interactions with the databases.

You do have to remember that the OS and SharePoint will take up to 20GB of that space.  If you don't put and search index files, SQL logs or anything else on that drive, you should be okay.  Though it is difficult to be certain without running a test.