Long time lurker, first time I've posted.
OP, get a TI-84. If you are considering a Mehanical Engineering major then you'll need one. The guy above me mentioned the TI-89, while this is a better calculator, it has a CAS which is a built in algebra system and many colleges will no longer let you use them in a math class, so your $120 calculator becomes an expensive paper weight. Also, you tend to end up using it as a crutch to take derivatives and do algebraic stuff, then when you get into classes where you need to take derivatives more complicated than Calc 1, it will give them to you, but in a screwed up way it's completely apparent it came from a calculator instead of doing them yourself. Learning to do them by hand is necessary, or you'll have a very tough time in Calculus 2 and 3.
I am a Junior in Electrical Engineering, I have one math class left, Differential Equations, and none of the class before that would allow a TI-89.
Get a TI-84, I can't stress it enough. The professor will be using one along with 99.9% of the rest of your Engineerig peers. You'll get to a point where the professor is teaching you to punch something into your calculator to look at the graphs and they will be using a TI-84. So if you're using anything else their directions won't work for the calculator you're using. An example of this is Fourier Series in Calculus 2. You'll be graphing multiple function of odd derivatives and see the convergence, at that point you'll want some direction. And hitting back on the TI-84 for Fourier Series, get the new TI-84 Plus CE. It's rechargeable with full color graphing so you can graph multiple functions and see what one is which, which will be very helpful in Calculus II, although not a necessity because I made it through the whole thing with a plain old non-color screen.
You can get a TI-84 Plus CE at Walmart for $110 ish, and the regular non-color for $10 less.