I'm an engineering student and the calculator that every student uses is the HP48 series. They take some time to get used to but once you do you will never go back. They are excellent calculators. The HP49 series recently came out but they're a bit pricey. Personally, I have trouble using anything else cause I love my HP so much.
The TI calculators are good too. If you decide to go with TI, DO NOT get the TI-86. The TI-89 is much better even though it is more expensive. If you buy the 86 I guarantee that you'll be buying an 89 within a year. The 86 is fine for high school and first year math, after that you'll need something better. You're better off buying the 89 now, it'll be cheaper in the long run. My TI-86 can't do half the things my HP48 can.
You might be better off going with TI just because the HP48/49 calculators are geared more towards engineering but they're excellent calculators none the less.
EDIT: troubledshooter is right about the TI-89, and that a lot of professors don't let students use calculators on tests. Engineering students usually get to (lucky me) but most others don't.