At my uni they are very strict with calculators, we are only allowed one specific type which has to have an "approved" sticker put on, and we have to put them all out so they can be checked at the start of an exam.
I do happen to have a graphing calculator but I don't use it much at all - only if I am doing a question involving graph sketching on a question paper and want to check I've got it right (because the answers at the end of the papers have the numerical answers, but not the graphical ones). If you're expected to do anything fancy and graphical in your coursework you'll get taught to use a program like Matlab to do it. Also it helps to use your exam-approved calculator all the time because then you become familiar with it and know it inside out by the time of the exam!
Personally I would wait until you're there and buy the approved one. In our case we could buy our calculator, drawing kit and data books in one go from the department on our first day there.