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 this
list.
How-to
Limiting RAM on OpenBSD
edit /etc/boot.conf
add: machine mem =512M
reboot
Limiting RAM and CPUs on Linux machines with the GRUB bootloader
edit /etc/default/grub
add/edit line GRUB_CMDLINE_LINUX="mem=512M maxcpus=1"
run grub-update
reboot
Limiting RAM and CPU in plan9
In plan9.ini
*nomp=1
*ncpu=1
*maxmem=512M
Recommended software
This is a list of software observed to be used by the community.