shuuhen

If you list the specs of the machine you intend to use for the new operating system(s), I can try give you a slightly better recommendation.

What I'd recommend at this point is use something like Gentoo or Arch for your workstation use. Both have a more in depth install, but you learn more about the system you'll be using. Then on separate or virtualized machines you could try to set up various servers, network them, handle upgrades, VPNs, etc. Plenty of guides are available on sites like BSD Guides, HowTo Forge, tldp.org, etc.

Pretty much any of the operating systems I listed should be good experience, but for server use you might do one of the *BSDs and either CentOS or Debian. If you have plenty of extra time on top of that, consider Solaris.

You could do a lot with only one or two machines if they can handle virtualizing some operating systems. You don't need all of them on at the same time, but it saves on physical hardware/space and energy. I haven't used Xen yet for virtualization, but I plan to since it looks like a powerful option for the OS's that support it. Virtual Box is free and fairly easy, but doesn't support 64-bit (last I checked anyway). VMware's products have worked fairly well for me - Server and ESXi are free. Of the two, I've only used Server, but it has helped me with both personal and work projects before. For Macs there are some additional options that cost a little, but work pretty well.