If I remember correctly, I didn't need anything more than the normal scientific calculator functions on the ACT, and I got a 30. I think it's kind of silly to buy an 84 over an 89 just so you can use it on the ACT.
Could you borrow a TI-84/83/81 from someone else to use on the test? If not, I really think you'd be fine with a plain ol' scientific calculator.
PS: TI-89's are really nice (and if you want a little more memory, go for the TI-89 Titanium)
