Are you going to use PowerPivot in your SharePoint 2010 farm?

Then make sure that you meet the following SQL Server 2008 R2 requirment that will have a great impact on your SQL Server license cost:

"You must have installation media for the Enterprise, Evaluation, Developer, or Data Center editions of SQL Server 2008 R2. "

That means that PowerPivot on top of SQL Server 2008 R2 Standard Edition is NOT possible.

Install PowerPivot in a new farm

Install PowerPivot on an existing farm

Other comments:

• PowerPivot for SharePoint adds SQL Server 2008 R2 Enterprise files to your SharePoint application Server