What version of SQL 2005 do I need to run on a 64 bit 2003 server

I have a 64 bit 2003 VM setup and I want to install SQL  2005 Enterprise but the only 64 bit version of 2005 enterprise that is the extended version and when I try to install it I am told that it is not compatible with the version of the OS I am using:server 2003, x62, SP2.

1) what version of SQL 2005 Enterprise will work on 2003, x64, SP2?

2) What does the term "extended" mean in the context of an SQL server install?

Thanks,

Chris Schene