Not really that way, depends on the database activity, the portion of the DB that is heavily used, the workload and a whole bunch of other things.

For what to leave for the OS, see chapter 4 of http://www.simple-talk.com/books/sql-books/troubleshooting-sql-server-a-guide-for-the-accidental-dba/

For calculating memory requirements, this may help: http://www.sqlskills.com/blogs/jonathan/post/How-much-memory-does-my-SQL-Server-actually-need.aspx