<p>The TI-83 and the TI-84 largely have the same functionality. The major differences are the included applications (some like them, some don’t), more space for holding applications (some people care about this, some don’t), and the faster processing speed. For most calculations, you won’t notice the processing speed (how hard is 2+2 anyway?), but in calculus, there will be times when you’re using some of the allowed features (see Nimrodel’s post above for more on this) where you’ll sometimes save as much as half a minute on the performance of the calculation.</p>
<p>One important piece from Nimrodel’s link is the following statement from the AP Website: “Care is taken to ensure that the exam questions do not favor students who use graphing calculators with more extensive built-in features.” A TI-89 simply won’t help you extra on the AP exam.</p>
<p>To me, the speed differential is probably worth the extra $20-$30 or so to get the TI-84 Plus, even if the other features aren’t worthwhile.</p>