If you've already finished studying and are just waiting for your scheduled date, make sure you come prepared with only the best calculators for the SAT. I can't sway you to pick the Casio FX 9750gii or the uber-popular TI 84, mainly because there isn't a single calculator for everyone. The best SAT calculators for you (who is just going to use this tool for the SAT) might not be the best for your friend who is planning to study advanced math in college after taking the SATs.
Choose one calculator from the list of 7 I highly recommended above and you're halfway there. All you just need to do is break a leg on the actual SAT test day, use your chosen calculator to answer the mathematics section, and finish on time.