Glynne_smith,
That is a great question! For many of the hard drive configurations in the Hard Drive Configurations for SQL Server tip the TempDB database and log file would reside on the System Database disk drive.
One item that was not covered in that tip is actually splitting the TempDB database (and transaction log) to a separate RAID 1 set when the TempDB database is used frequently. Depending on the application, TempDB may or may not be used heavily. For the circumstances when it is used heavily, then splitting the TempDB database (and transaction log) may be a performance improvement.
Let us know if this answers your question.
Thank you,
The MSSQLTips.com Team