I second the above, but I am trying out a few things.
Consider the setup that Darren and the team made for their VM Server, Proxmox, a virtualization alternative to VMWare. I've tried out ESXI and while it is decent to mess around with, well, i dont like dealing with temporary licenses, and a somewhat anoying setup. So I choose VMWare Workstation as my virtualization platform, at least until i get home and I can try proxmox.
I would build out in your VM software, a full windows domain, including Windows 7, Server 2k8r2, set up DNS servers, DHCP, AD, WSUS, NAP, ect. This allows you to first practice your MS skills. Eventually I would look into adding some security devices, open source firewalls, IDS, ect. This lets you learn some defensive technologies, including packet analysis.
Lastly, set up BT, if you are particularly adventurous, you could try to install the tools on ubuntu and get them working, or you could just run BT5. Do some tests with different configurations, (What happens if you allow DNS Transfers?)
Specificly, I can suggest a few things to throw in there:
Security Onion: out of the box IDS
Metasploitable
Damn Vulnerable Linux
Heorot.net live cd's (the website i think isnt up anymore, i think its been transfered to their training program, but you should be able to find the ISO's.): Hcking Challenges with preconfigured vm's