Just get a TI-83, 84, 85 or 86. The 89 is overkill and sometimes banned from tests , etc. There is very little difference in power or use between the 83 and 84 or 85 and 86, it's a matter of preference. I prefer the 86, but the 83 will probably be easiest to come by used as it is the most popular.
People talk up features and such, but all you need is a calculator that can do addition, subtraction, division, multiplication, powers, roots, logarithms and graphing (maybe matrices, too, if you swing that way). Anything more powerful than that (say a calculator with built-in physics functions) can result in temptation to cheat, thereby keeping you from learning the things you need to learn. Don't use any helper or solver applications that you didn't write yourself.