If you want to get really cheap, older editions of the books so you can have them as a reference, buying them early is helpful because it gives you time to buy them online and get them shipped. I personally didn't use my textbooks more than a few times my first year (with the notable exception of anatomy), so I was glad that I bought the previous editions. The information is mostly the same as the latest edition, and certainly the concepts are. It's usually just a few new figures or a paragraph or something is added to the chapter so they can sell a new book. Usually you can get the second most recent edition for $5-$15 if you look on Ebay, half.com, and Amazon. If your school gives good lecture notes, you might not ever use your books. If you used your books a lot in undergrad, you might be glad you have them.
In retrospect, the only books I would have bought would be Netter's and Rohen's Atlases for anatomy. Those I used A LOT.
edit: and BRS Physiology. That helped a lot and I plan to use it for the boards anyway. Oh, and Mosby's. That was useful.