SQL Server 2005 Indexing

1) Most of our PK's are non-clustered. In this case when should we go for a clustered index.

2) Apart from Primary Keys which are indexed by default, I am going for single column indexes for foreign keys & other
columns which are involved in WHERE, GROUP BY, HAVING & ORDER BY.

3) When is a covered index good to have i.e. a composite index on entire select list.

4) When should we go for a index on HERE, GROUP BY, HAVING or ORDER BY columns followed by select list columns.

5) Are functional indexes allowed in SQL Server i.e. when column is in function like ABS/MAX etc. If so, what is the DDL?

6) When data has been encrypted and we decrypt while selecting, how to make sure that index gets picked.