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.