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

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

Keep 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.