I might second the above arguments of getting another hard drive instead of another machine.
I've always had problems setting up dual/multi booting. If the machine crashes and you don't have a backup it is a lot worse than a single install.
However with all the cheap external drives you can get nowadays you can set the external drive up as a primary with an OS on it.
Then change the bios to let you boot from USB or eSata. Then select to boot form the external drive at the start up.
I installed AROS once on some older hardware without any problems, I haven't bothered to try again, but will soon. If it can do everything Linux can, then it will work nice for a basic system.