I only have 2 spare machines and both are very old, but I really want to set up an environment so that I can start practicing security. One of these machines has a Pentium 4 processor and 512 MB RAM; the other is older (and I don't know the specs off the top of my head).
I want to set up a VM environment on these machines, if possible.
What would be the best way to do it, to keep the rest of my network secure? Would a better option be to leave them as they are (Windows XP box and a Ubuntu 10.04 server) and change some settings/install vulnerable software (burp suite, web goat, etc)?
What other issues should I consider, from a security point of view?