TI-83 plus SILVER edition for around $140 is probably the most reasonable for university students. It is good for all math classes, including Algebra, Geometry, and Calculus. - also useful for science and technology classes. If you want to spend more/less money: TI-83 Plus for $100, TI-89 for about $200, and then newest TI-84 Plus for $400.
Disclaimer: All of the capabilities I describe below are built-in to the TI-89 as it comes out of the box -- not requiring the download or installation of any extra illicit software.
Graphing is a small part of the TI-89's capability. It is a robust CAS (Computer Algebra System). Enter most any algebra or calculus problem and receive a symbolic result. Factor and expand polynomials. Take derivatives and integrals. Substitute expressions into one another, as you would on paper. Manipulate variables in a symbolic way, like in Mathematica. Output is formatted in "pretty print," so solutions look identical to the way they're printed in a textbook.
It is no mere visual aid -- it will solve the whole problem for you in one step. Studying with this calculator takes extra effort: since the answer is a keystroke away, you have to work with much more discipline to actually learn your material. I make it a point to solve a problem by hand first and use the TI-89 only to check my work; however, even doing this, you can lose the sense of confidence in your own work -- when it doesn't feel correct unless the machine says so.
All in all, the TI-89 is an incredible problem solving tool. Most of my professors underestimate its capability, and even my peers who own the calculator hardly know what it will do. Treat it like you would an integral table: the table is not a learning tool, but later you will require its convenience when solving more difficult problems.
i never used a graphing calculator untill i got to my Precalc 2 class. The professor required one or we would fail, so i went out and bought a TI-83, used it that semester and gave it away. never needed it after that.
I have owned and used the TI-83, TI-86, and TI-89.
The 89 is too difficult to use and gives uncertain forms. Of course, it is the best for some problems, since it gives "a lot" of answers in closed form. It has a very small display and so is hard to read. Its use is too specialized.
The 86 has been around the longest, and in my opinion is the most useful and effective calculator you get for most any purpose, including graphing.
I don't see the 83, silver edition included, as having much to recommend itself one way or the other. It is sometimes faster than the TI-86, but is more limited in its range of operation.
However, I have had friends who complain that the TI series is too difficult to use, and that is because they do not want to study the manual WHICH IS ESSENTIAL TO USING THE CALCULATOR. If you want something that does not require learning necessary "sentances" for input, that resembles how you would work the problem on paper, well, try somewhere other than TI. The TIs are really a form of computer programing, rathan than intuitive calculation.