A Pentium II or Celeron system with 128 MB of RAM is probably a bottom-line configuration that may yield slow yet usable system with a standard lubuntu desktop.
14.04 32 bit ISO require your CPU to have Physical Address Extensions, or PAE. "PAE is provided by Intel Pentium Pro and above CPUs, including all later Pentium-series processors (except most 400 MHz-bus versions of the Pentium M)." - If you have an error with Celeron M reporting "NON-PAE CPU" and would like to install Lubuntu 14.04, please see this page.
For PowerPC, it is known to run on a G4 running at 867MHz with 640MB RAM.
For Intel based Macs, lubuntu should run on all models.
For common issues with Ubuntu, please refer to the Ubuntu release notes.
Head over here to get it.