Hello,
I have set up a test environment in which I have a single server with 4gb RAM currently running Active Directory Domain Services with only 2 clients and it runs fine (only using 990MB). I am intending to install SQL Server on it and limit it to 2gb ram. As well as these, I would very much like a domain-based DFS Namespace (I am not bothered about file replication) but will all three roles be able to run efficiently with only two clients? I don't want it to be really laggy taking ages to process logon requests and SQL queries etc. I would rather have SQL and AD run fine than have a DFS as well and have them really slow.