make things simplier,
you cannot use standard CALs for 2008 AD only, because you need exchange CALS, SQL CALS and etc to make up a standard/premium EBS CALS.
a standard CALS includes, Messaging server, management server and security server (and all CALS required to access them)
the difference of standard and premium CALS is it also included the 4th server SQL server CALS.
thats why the price is much higher.
If you only need email and sql, you can go SBS 2008 premium with 2 servers.
the limitation of SBS 2008 is you only can grow up to 75 users where EBS can grow up to 300 users