What OS should I use?

I'm looking to get operating system software for a new server. I am planning on running various dedicated servers, including a VOIP Server, several game servers and possibly an IRC.
I have looked into multiple platforms and servers, and I have a list of the few I would like:

Windows 7
Windows Server 2008
Windows Server 2012.

My question is this: Should I be looking for server OS? Or just plain Windows? Also, can I still run multiple dedicated servers from 1 CAL?

Anonymous2012-11-11T13:34:25Z

You can use any. Preferably not Windows 98, Vista, 8 or 2012 Server. Windows 7 or Windows 2008 Server standard would do the job nicely. You don't have to worry about CAL as that is just computers in your local network. If your planning on running multiple servers its recommended you have lots of RAM (+16Gb please :) and port forwarding on default ports for simplicity (unless there taken) Have fun!