If your interest is in becoming a Linux administrator one skill you must have is the ability to network servers and understand the interaction between servers. You will also need to know how to build servers using RAID, LVM and optional file systems. These skills can be practiced on a virtual network of practice servers. You can build a training network using one workstation. I use several options for building a training network; VMWare Player, OpenVZ and VirtualBox.
VMWare Training Server
Directions on How to Build It Here
You can build a network using VMWare Player and a number of virtual servers. VMWare player is free to use and by using sites like easyvmx.com you can build your own instances for testing. These images you create can have multiple drives so that you will be able to gain practice with RAID and LVM. These skills are essential for a Linux admin. VMWare does the best job at giving you access to actual hardware. VMWare player works well on a Linux or Windows machine but it does use a lot of resources. With a system having 2-3 GB of RAM you will only be able to run two instances at a time. Your virtual machines should have 400-500 MB of RAM to work effectively.
OpenVZ provides a way to get the most out of the hardware you have. Because the virtual machines use the same kernel as the host you save a lot on resources. So if you need to run 10 virtual machines at once, this is the best choice. OpenVZ does not allow a graphical interface but for servers it works great. I have been able to build as many as 15 concurent virtual machines on one box with 1 GB of RAM. You cannot do anything like that with VMWare or VirtualBox.
Virtual Box Training Server
Directions for Building HERE
VirtualBox is always my choice on a workstation. VirtualBox seems more stable, user friendly and efficient than VMWare on a Linux workstation. It is a simple process to download an iso image and then create multiple instances. I often will have several servers and desktops running at the same time and essentially create your own network. This is a great way to use the instances to interact with each other. One thing to keep in mind is that you will want to increase RAM so it runs correctly. My workstation has 6 GB of RAM so I do not worry about resources when running a number of servers or desktops.
No doubt about it one the the greatest learning tools is a virtual network.