Where the pk is is utterly irrelevant. It's the clustered index that you need to be concerned about (the pk is clustered by default, but it's just a default)
Maybe take a read through this series
http://www.sqlservercentral.com/articles/Indexing/68439/
http://www.sqlservercentral.com/articles/Indexing/68563/
http://www.sqlservercentral.com/articles/Indexing/68636/
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
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