Like Viro said: The 64bit'ness of Tiger won't do a non-G5 Mac any good. It won't do any harm, either, because it'll be optimised for both 32bit and 64bit Macs.
Dashboard is 'good' alright, but it'll certainly depend on how you're going to use it. If you'll have like 5 widgets open on the Dashboard, that's going to be quite informative. I fear that users, however, will clutter their Dashboard and thus make its main benefit (information at a glance) vanish...
There's certainly going to be a lot of Dashboard widgets available when Tiger will be available. The important thing will be to choose what you really want to have readily available as information on your Dashboard. Do I need my weather forecast there? Maybe. Do I want a Gmail notifier now that I can also use POP3 for that account? Probably not.
What I _can_ say is that the technology with how the widgets drop in and fade out is simply put gorgeous. It's certainly going to be a great way to show off Mac OS X' technologies in front of those PC using friends.
So: Will Tiger be worth it for your eMac G4? - I'd say yes. But while I _do_ expect some performance increases, I guess it's mainly going to be the added functionality that will drive people to Tiger. Sure, you can get Konfabulator to do mainly what Dashboard does, and you might find other replacements for other new features in Tiger. But the package will make it very, very good. And Spotlight is going to change the way how you work with a computer.
That being said: You chose the wrong thread for your question, and I'd like to urge you to use the 'changes' thread for such questions and not the one about stability in the future.