Forget the hack that I posted earlier. A quick review of various threads about Tiger on the 800MHz iMac seems to indicate that the video card in that model has problems keeping up with demand, and that iChat in particular may have some real problems in the upgraded system. Since iChat is the main reason that my inlaws use their Mac (along with Mail), I think we’ll hold off.
Details: MacOSXHints on experiences with hacked Tiger installs; sleep problems; sleep problems 2.