You get a list of "required" books durring oreintation. I would hold off until then before you get any. You get paired up w/ a second year for part of a day and they will tell you which ones you absolitely need and which ones are only so so. Some of them you deffinetly do not need.
Sometimes on the web board people are selling old books. Also there is a used book sale for SOMA members if/when you sign up. I think the best bet is to get them online if you get them new, because its cheaper. Amazon.com has decent prices.