Getting one's hands on a real old machine may not always be viable.
Since the general idea is to be using a computer with limited
resources, there are ways this can be achieved through software methods
on modern hardware.
Perhaps you're only looking for tips on what software to use. See the list
below.
How-to
Limiting RAM on OpenBSD
* edit /etc/boot.conf
* add: machine mem =512M
* reboot
Limiting RAM on Linux machines with the GRUB bootloader
* edit /etc/default/grub
* add/edit line GRUB_CMDLINE_LINUX="mem=512M"
* run grub-update
* reboot
Recommended software
This is a list of software observed to be used by the community.
