I am setting up Exchange 2010. My question is what CAL's do I need. I see that there are 2 types enterprise and standard. Enterprise seems to be cheaper. Are these an upgrade from Standard? Can you just buy the enterprise CAL's without buying the standard?

This would be helpful http://www.microsoft.com/exchange/2010/en/us/licensing.aspx

The enterprise CAL is additive to the standard CAL meaning that you cannot buy the enterprise CAL only. If you need the enterprise CAL you will also have to buy the standard CAL. The link that Khoj listed will give you what features are included in each CAL.Tim Harrington - Catapult Systems - http://HowDoUC.blogspot.com