License Schema on SQL Server 2005, 2008 and 2012

Hello Friends

i have a physical server that has 2 processors with 10 cores each one and because the hyperthreading technology (i guess) in my os appears 40 cores.  i need install SQL Server 2008 Standard and i need determinate what kind of the license schema applies to this SQL Server. by processor or by core?

Aditionally in SQL Server 2005 and 2012 is the same schema?

Thanks for your help and greetings.

Alex