If you like to explore or test, it will run very fine on 2GB ram with i3 or i5
Windows server is not mandatory, in fact if you ask me I will not advice you to go with windows server, your DM should be hosted in more secure OS
You can install alfresco on 64 bit Linux OS also, and it is advised for production purpose, rock solid.
I have few CE DM running on CentOS 64 bit with 4GB ram