Below links have all the information that you need to know before installing\upgrading to Backup Exec 15.
Backup Exec 15 Compatibility
Backup Exec 15 Software Compatibility List.
Backup Exec 15 Hardware Compatibility List.
Backup Exec 15 Documentation
Backup Exec 15 Administrator's Guide.
What's different in Backup Exec 15
Backup Exec 15 Command Line Interface (BEMCLI) Help
Backup Exec 15 Tuning and Performance Guide
Backup Exec 15 Trialware, Licensing and Support
How Agents and Options are Licensed in Symantec Backup Exec 15 (Licensing Guide)
Symantec Backup Exec 15 Capacity Edition
Symantec Technical and Customer Support Contact Details
Backup Exec 15 Upgrade and Fresh Install
About upgrading from previous versions of Backup Exec to Backup Exec 15