I am testing several servers in lab environment and I am wondering what is more memory hungry. a DC01 with AD on or a MEM01 with several roles installed such as DHCP, VPN, deployment etc
Is Active directory more demanding then the others you think?
AD is pretty light on RAM usage. It's generally best practice to install DNS on the same server, but it's not required. The memory footprint of both will vary with the size of the AD database or DNS database (which is AD backed), respectively.
For a test environment, both will take up very little memory; a 2008 R2 Core box running AD & DNS would be very comfortable with 512 MB of ram, you might be able to skimp all the way down to 384 MB without problems. If it's not a Core install, you might still get away with 512 MB, but 768 MB is a better idea.
DHCP and Remote Access are also quite light on RAM usage. Deployment services take a little more, but still not much. In all these cases it will depend on the usage.