I never run full screen except when checking questions like yours, but I tried it again, and it "just works" here - Command Tab or Command-H both hide the full-screen Civ3 display, allowing me to see my normal desktop. You don't get an application selection ribbon going this way. Command-Tab then gives me the usual app switching ribbon where I can reselect Civ3 and its display reappears. I'm running an Intel MacPro with OS X 10.4.10, but I doubt if your PPC CPU and older OS would affect this function.
If you want to have access to other Mac apps, as I do, have you considered running Civ3 in a window? I find it far more flexible. And as I have two screens, I have Civ3 on one screen, and the rest of my world on the other, which also has the dock and menu bar.
Re. processor utilisation, there's a checkbox in the start-up dialog to reduce it (Command-double-click the Civ3 application to open the dialog). In Rev A I think that is supposed to make laptop operation cooler. Have you tried it?