What version of Exchange do I need? Enterprise or Standard?

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Feature	Exchange Server 2003 Standard Server	Exchange Server 2003 Enterprise Server	Exchange Server 2007 Standard Server	Exchange Server 2007 Enterprise Server	Exchange Server 2010 Standard Server	Exchange Server 2010 Enterprise Server
Mailbox Databases	1-2	3-20	1-5	6-50	1-5	6-100
Database Storage Limit	75GB	None	16TB	16TB	16TB	16TB
Single Copy Cluster	No	Yes	No	Yes	No	No
Local Continuous Replication	No	No	Yes	Yes	No	No
Standby Continuous Replication	No	No	Yes	Yes	No	No
Cluster Continuous Replication	No	No	No	Yes	No	No
Mailbox Resiliency	No	No	No	No	Yes	Yes

\nKeep in mind you also need client access licenses or CAL's.  The are sold in standard and enterprise edition.  The enterprise however is just an add-on to the standard license.  So for one enterprise user you must have a standard CAL and enterprise CAL.  The main difference between the two is unified messaging which enterprise gives you the rights to.