Symptom
The Sleep command in the Apple menu appears dimmed (or "grayed out"). The computer does go into sleep mode for a period of inactivity as defined in the Energy Saver pane of the System Preferences application.
Solution
Some PCI cards may prevent Mac OS X from entering sleep mode. Your computer should sleep when the PCI card is not present.
Note: Apple-original video cards installed in the dedicated video slot do not cause this symptom. If you have additional Apple Configure-to-Order video cards installed in other PCI slots, you may experience this issue.
Even if you have a PCI card that prevents full sleep, you may still use other Energy Saver features. You may set system sleep to "Never" in the Energy Saver pane of System Preferences while maintaining monitor sleep and hard disk spin down.