"Worth the money" is a relative term, it all depends on how much you have to spend. But will it save you a shitload of time doing various things? Yes. It's likely to be able to do far more than you could ever need it to do. You can load things on to it, so yes, it has that for you.
The TI site has tons of files that you can download and then load them on. A quick google search found this page which has all sorts of downloadable zip files that contain calculations you can load on. Check through it to see if it has what you need.
You can download the manual for free (click her to begin downloading PDF) so check it out, that should help in your decision-making process as well.
I have no experience with the ti-89 so I can't say how much better it is, or whether it's worth the extra money.
You may want to try out an emulator first if you don't have much experience with the ti-80 series calculators.
wabbitemu is one example that will load up the ti-83 (I don't think it does 84 yet) on your computer screen so you can try out all the wonderful things it can do, and see how easy you find it to be. There are some things that are not as intuitive as you would think, but for the most part it's pretty good.
The new calculators they're trying to push are the TI-Nspire family, and they can do A LOT of shit. But more importantly, they're much, much more user-friendly. The newest versions are even in full colour. I've tried the black-and-white version of these and it really is quite impressive. But of course, they cost a lot more money. Still, something you may want to check out depending on how much you're going to use it (i.e. after school or only until you get your degree).
OK I've said enough for a first post, any further questions on what I've said, feel free to ask and I'll give as much detail as I can.
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my ti89