From a teach yor self linux and security point of view. I would take the copy of Knoppix STD, that i haveand install on my spare machine.

Knoppix is a live version of linux that can be run from a cd rom, which is exactly what i am playing with at the moment. I have not got a spare pc at the moment so i have not installed it.

Knoppix STD is a security inhanced version of Knoppix with a list of tools that is quite impressive.

authentication
encryption
forensics
firewall
honeypot
ids
network utilities
password tools
servers
packet sniffers
tcp tools
tunnels
vulnerability assessment
wireless tools

Some more info here: http://www.knoppix-std.org/

If i had a spare box thats what i would do learn and have fun at the same time.

I only mention Knoppix because it's the only version of linux that i am familiar with. there may be other disto's equally loaded with security tools, i'm sure some of the nix gurus will but us straight on that point.

good luck and have fun, what ever you decide.

Jinxy