If 8GB is the maximum your laptop can run, then I would create one VM and using Virtual Box or VMPlayer create a stand-alone Widnows Server 2008 VM and assign it 4GB of RAM. The Evaluation Edition will work for 180 days and when you get toward the end there is a command that will extend it out for another period of time. You can find more information in the Microsoft site. After that I'm afraid you will have to start over, but if you backup your databases and anything else critical off of the VM first, you can make a new VM and practice your disaster recovery strategy. 😉

For the failover clusters and perhaps other high availability exercises, you'll need at least three servers. I would create a domain controller and give it 1GB since the authentication duties will be light in your practice environment. Create two member servers with 2GB each. Not a lighting fast setup but you can get mirroring, AlwaysOn and cluster failovers to work.

I am assuming this is the only computer in the house. If there is another, even an older one with only 4GB of RAM you can run a VM there and access it through your home network.