NB_CLT_6.5.6.
Win2008SC.x64.exe is for Windows 2008 Server Core ONLY.
Assuming they're all X64, I think the one you REALLY want for your existing servers AND clients is this one:
Release Update NB_6.5.6.winnt.x64.exe provides fixes to the Veritas NetBackup (tm) Enterprise Server / Server 6.5 on x64 Windows server and clients. It also contains fixes for NetBackup Add-on products and Database Agents.
http://support.veritas.com/docs/351563
Good luck!