Want Adventurous. Mountain Lion on on old unsupported macs
http://osxdaily.com/2012/02/18/install-os-x-mountain-lion-developer-preview-on-old-unsupported-macs/
http://osxdaily.com/2012/02/18/install-os-x-mountain-lion-developer-preview-on-old-unsupported-macs/