Tips on Optimizing SQL Server Clustered Indexes

http://www.sql-server-performance.com/tips/clustered_indexes_p1.aspx
As a rule of thumb, every table should have a clustered index. Generally, but not always, the clustered index should be on a column that monotonically increases--such as an identity column, or some other column where the value is increasing--and is unique. In many cases, the primary key is the ideal column for a clustered index.
If you have any experience with performance tuning SQL Server 6.5, you may have heard that is not a good idea to add a clustered index to a column that monotonically increases because it can cause a "hotspot" on the disk that can cause performance problems. That advice is true in SQL Server 6.5.
Normally, "hotspots" aren't generally a problem. You would have to have over 1,000 transactions a second before a "hotspot" were to negatively affect performance. In fact, a "hotspot" can be beneficial under these circumstances because it eliminates page splits.
Here's why. If you are inserting new rows into a table that has a clustered index as its primary key, and the key monotonically increases, this means that each INSERT will physically occur one after another on the disk. Because of this, page splits won't occur during INSERTs, which in itself saves overhead. This is because SQL Server has the ability to determine if data being inserted into a table has a monotonically increasing sequence, and won't perform page splits when this happens.
If you are inserting a lot of rows into a heap (a table without a clustered index), data is not inserted in any particular order onto data pages, whether the data is monotonically or not monotonically increasing. This results in SQL Server having to work harder (more reads) to access the data when requested from disk. On the other hand, if a clustered index is added to a table, data is inserted sequentially on data pages, and generally less disk I/O is required to retrieve the data when requested f