What version of Exchange 2010 do I need? Enterprise or Standard?

Feature Standard Server Edition Enterprise Server Edition
Mailbox Databases 1-5 databases 6-100 databases
Roles Based Access & Control Yes Yes
Transport Resiliency Yes Yes
Remote PowerShell Yes Yes
Online Move Mailbox Yes Yes
Web-based administration Yes Yes
Mailbox Resiliency Yes Yes

Keep in mind you also need client access licenses or CAL’s.  They 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.