OK, you used Software Update, right? (You can access it from the Apple menu). Click update, and it should update everything that needs updating, including firmware updates.
You should check the firmware version. Choose Apple menu -> About this Mac. It should say 10.4.7. Earilier versions will not work.
Did the software update fail, and you still don't have the 10.4.7 update? Head over to Apple's site to manually download and install the update:
http://www.apple.com/downloads/macos...1047intel.html
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