Hibernate preference pane will enable set switch between different sleep states on your Mac:
hibernate & sleep
If you own and PPC Mac with 1GHz or more chances are good that Hibernation will work on your machine. Some machines do need the NVRam patch. That means that there are some values then stored in the no-volatile ram of the computer to say it’s hibernate capable. When the os boots up it checks for those values. The software currently tests for machines>=1 GHz to enable NV-Ram patching. It is known that some machines don’t sleep at all, some won’t start up their fans when hibernation is enabled. In case something goes wrong disable hibernation by resetting the NV-Ram (Hold down CMD-ALT-P-R when booting).
In any case use this software at own risk. There might be a reason why apple enables hibernation only on newer machines.