http://www.bigwords.com is my favorite -- textbook search engine!
If you know for sure you're going to be taking a certain set of classes, go ahead and order the books ASAP if you want... but know that a lot of classes don't post the book they're using on the class website until late August -- and a lot of classes (particularly HASSes) have Stellar sites (
http://stellar.mit.edu) which won't let you access them until some time after registration day.
The Coop will probably get books in around Aug 15-20, so you can head there when you arrive for Orientiation, write down the ISBNs of the books you need, and see if you can get them significantly cheaper online.
But I second mitsoph's plug for upperclassmen, so long as the book they used is the book which is still being used. The textbook companies love to update the editions so you have to buy new instead of used.