ActiveDirectory is neccesary to install windows-base vCenter 5.1 ?
No, You can still install vCenter without AD. But if its part of a domain it makes your life easier as you can have useres imported from AD and do not have to worrk about creating local users and stuff
4GB memory and 2core CPU is neccesary ?
How many hosts will you have? Do you have a database on the same machine? If yes then go with 4GB or 2GB should work just fine.
Active Directory is not a requirement for installing SSO, if you are doing a fresh install the only requirement is that the hostname(FQDN) is DNS resolvable, i.e., the forward and reverse DNS lookup should work. However if your SSO has to work with Active Directory i.e., you are planning on using the users and groups from the active directory to assign permissions to vcenter server and if your machine is joined to the AD domain there is a convenient way to add the active directory as an identity source(Please see SSO Identity Sources) with Active Directory (Integrated Windows Authentication) which makes it easier to enumerate the trusted domains to assign permissions in vcenter server for users and groups in the trusted domains.