In addition to my previous post about the SQL Server 2008 editions, here’s another comparison sheet for the 2008 R2 Editions:
Features Supported by the Editions of SQL Server 2008 R2
Many of the differences were kept intact, except for the addition of the Datacenter Edition.
If you want to use the new PowerPivot features, you’ll be forced to buy the Enterprise Edition though.
SQL Server 2008 R2 Pricing
For more information about pricing and licensing, check the SQL Server 2008 R2 Licensing Quick Reference Guide.