TI's are the best graphing calcs out there. My TI-89 gets me through a LOT. I'd avoid the 92, though. It's the same calc as an 89, but it has a QWERTY keyboard for easier programming. Trouble is I hear most profs have banned it, but have not banned the 89 because they don't realize it's got the same power. It's a stealth 92 basically
If you can swing it, go with an 89, but the 83 would do well as well. Hell an old TI-85 would get you through pretty much anythign you needed.